HLREST: Add x-pack-info API (#31870)

This is the first x-pack API we're adding to the high level REST client
so there is a lot to talk about here!

= Open source

The *client* for these APIs is open source. We're taking the previously
Elastic licensed files used for the `Request` and `Response` objects and
relicensing them under the Apache 2 license.

The implementation of these features is staying under the Elastic
license. This lines up with how the rest of the Elasticsearch language
clients work.

= Location of the new files

We're moving all of the `Request` and `Response` objects that we're
relicensing to the `x-pack/protocol` directory. We're adding a copy of
the Apache 2 license to the root fo the `x-pack/protocol` directory to
line up with the language in the root `LICENSE.txt` file. All files in
this directory will have the Apache 2 license header as well. We don't
want there to be any confusion. Even though the files are under the
`x-pack` directory, they are Apache 2 licensed.

We chose this particular directory layout because it keeps the X-Pack
stuff together and easier to think about.

= Location of the API in the REST client

We've been following the layout of the rest-api-spec files for other
APIs and we plan to do this for the X-Pack APIs with one exception:
we're dropping the `xpack` from the name of most of the APIs. So
`xpack.graph.explore` will become `graph().explore()` and
`xpack.license.get` will become `license().get()`.

`xpack.info` and `xpack.usage` are special here though because they
don't belong to any proper category. For now I'm just calling
`xpack.info` `xPackInfo()` and intend to call usage `xPackUsage` though
I'm not convinced that this is the final name for them. But it does get
us started.

= Jars, jars everywhere!

This change makes the `xpack:protocol` project a `compile` scoped
dependency of the `x-pack:plugin:core` and `client:rest-high-level`
projects. I intend to keep it a compile scoped dependency of
`x-pack:plugin:core` but I intend to bundle the contents of the protocol
jar into the `client:rest-high-level` jar in a follow up. This change
has grown large enough at this point.

In that followup I'll address javadoc issues as well.

= Breaking-Java

This breaks that transport client by a few classes around. We've
traditionally been ok with doing this to the transport client.

docs/java-rest/high-level/miscellaneous/x-pack-info.asciidoc

@@ -0,0 +1,64 @@
+[[java-rest-high-x-pack-info]]
+=== X-Pack Info API
+
+[[java-rest-high-x-pack-info-execution]]
+==== Execution
+
+General information about the installed {xpack} features can be retrieved
+using the `xPackInfo()` method:
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests}/MiscellaneousDocumentationIT.java[x-pack-info-execute]
+--------------------------------------------------
+<1> Enable verbose mode. The default is `false` but `true` will return
+more information.
+<2> Set the categories of information to retrieve. The the default is to
+return no information which is useful for checking if {xpack} is installed
+but not much else.
+
+[[java-rest-high-x-pack-info-response]]
+==== Response
+
+The returned `XPackInfoResponse` can contain `BuildInfo`, `LicenseInfo`,
+and `FeatureSetsInfo` depending on the categories requested.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests}/MiscellaneousDocumentationIT.java[x-pack-info-response]
+--------------------------------------------------
+<1> `BuildInfo` contains the commit hash from which Elasticsearch was
+built and the timestamp that the x-pack module was created.
+<2> `LicenseInfo` contains the type of license that the cluster is using
+and its expiration date.
+<3> Basic licenses do not expire and will return this constant.
+<4> `FeatureSetsInfo` contains a `Map` from the name of a feature to
+information about a feature like whether or not it is available under
+the current license.
+
+[[java-rest-high-x-pack-info-async]]
+==== Asynchronous Execution
+
+This request can be executed asynchronously:
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests}/MiscellaneousDocumentationIT.java[x-pack-info-execute-async]
+--------------------------------------------------
+<1> The `XPackInfoRequest` to execute and the `ActionListener` to use when
+the execution completes
+
+The asynchronous method does not block and returns immediately. Once it is
+completed the `ActionListener` is called back using the `onResponse` method
+if the execution successfully completed or using the `onFailure` method if
+it failed.
+
+A typical listener for `XPackInfoResponse` looks like:
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests}/MiscellaneousDocumentationIT.java[x-pack-info-execute-listener]
+--------------------------------------------------
+<1> Called when the execution is successfully completed. The response is
+provided as an argument
+<2> Called in case of failure. The raised exception is provided as an argument

docs/java-rest/high-level/supported-apis.asciidoc

@@ -53,9 +53,11 @@ The Java High Level REST Client supports the following Miscellaneous APIs:
 
 * <<java-rest-high-main>>
 * <<java-rest-high-ping>>
+* <<java-rest-high-x-pack-info>>
 
 include::miscellaneous/main.asciidoc[]
 include::miscellaneous/ping.asciidoc[]
+include::miscellaneous/x-pack-info.asciidoc[]
 
 == Indices APIs
 
@@ -181,4 +183,3 @@ The Java High Level REST Client supports the following Scripts APIs:
 
 include::script/get_script.asciidoc[]
 include::script/delete_script.asciidoc[]
-