Add API to execute SLM retention on-demand (#47405)

* Add API to execute SLM retention on-demand

This commit adds the `/_slm/_execute_retention` API endpoint. This
endpoint kicks off SLM retention and then returns immediately.

This in particular allows us to run retention without scheduling it
(for entirely manual invocation) or perform a one-off cleanup.

This commit also includes HLRC for the new API, and fixes an issue
in SLMSnapshotBlockingIntegTests where retention invoked prior to the
test completing could resurrect an index the internal test cluster
cleanup had already deleted.

Resolves #46508
Relates to #43663

docs/java-rest/high-level/ilm/execute_snapshot_lifecycle_retention.asciidoc

@@ -0,0 +1,35 @@
+--
+:api: slm-execute-snapshot-lifecycle-retention
+:request: ExecuteSnapshotLifecycleRetentionRequest
+:response: AcknowledgedResponse
+--
+[role="xpack"]
+[id="{upid}-{api}"]
+=== Execute Snapshot Lifecycle Retention API
+
+
+[id="{upid}-{api}-request"]
+==== Request
+
+The Execute Snapshot Lifecycle Retention API allows you to execute Snapshot Lifecycle Management
+Retention immediately, rather than waiting for its regularly scheduled execution.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-request]
+--------------------------------------------------
+
+[id="{upid}-{api}-response"]
+==== Response
+
+The returned +{response}+ contains a boolean for whether the request was
+acknowledged by the master node.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-response]
+--------------------------------------------------
+
+include::../execution.asciidoc[]
+
+