Flag in _field_caps to return only fields with values in index (#103651)

We are adding a query parameter to the field_caps api in order to filter out 
fields with no values. The parameter is called `include_empty_fields`  and 
defaults to true, and if set to false it will filter out from the field_caps 
response all the fields that has no value in the index.
We keep track of FieldInfos during refresh in order to know which field has 
value in an index. We added also a system property 
`es.field_caps_empty_fields_filter` in order to disable this feature if needed.

---------

Co-authored-by: Matthias Wilhelm <ankertal@gmail.com>

qa/multi-cluster-search/src/test/resources/rest-api-spec/test/multi_cluster/30_field_caps.yml

@@ -65,7 +65,6 @@
       field_caps:
         index: 'my_remote_cluster:some_index_that_doesnt_exist'
         fields: [number]
-        
   - match: { error.type: "index_not_found_exception" }
   - match: { error.reason: "no such index [some_index_that_doesnt_exist]" }
 
@@ -156,3 +155,38 @@
   - length: {fields.number:                             1}
   - match: {fields.number.long.searchable:              true}
   - match: {fields.number.long.aggregatable:            true}
+
+---
+"Field caps with with include_empty_fields false":
+  - skip:
+      version: " - 8.12.99"
+      reason: include_empty_fields has been added in 8.13.0
+  - do:
+        indices.create:
+          index: field_caps_index_5
+          body:
+            mappings:
+              properties:
+                number:
+                  type: double
+                empty-baz:
+                  type: text
+
+  - do:
+      index:
+        index:  field_caps_index_5
+        body:   { number: "42", unmapped-bar: "bar" }
+
+  - do:
+      indices.refresh:
+        index: [field_caps_index_5]
+  - do:
+      field_caps:
+        include_empty_fields: false
+        index: 'field_caps_index_5,my_remote_cluster:field_*'
+        fields: '*'
+
+  - is_true: fields.number
+  - is_false: fields.empty-baz
+  - is_true: fields.unmapped-bar
+  - is_true: fields._index

qa/multi-cluster-search/src/test/resources/rest-api-spec/test/remote_cluster/10_basic.yml

@@ -82,6 +82,10 @@
                       nested2:
                         type: keyword
                         doc_values: false
+  - do:
+        index:
+          index:  field_caps_index_1
+          body:   { number: "42", unmapped-bar: "bar" }
   - do:
       indices.create:
         index: test_index