### Description

Before ES 7.x it was possible to provide a list of fields by URL or BODY request for field_caps API.
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/search-field-caps.html
`POST _field_caps
{
   "fields" : ["rating"]
}`

Starting from 7.x this feature was removed, and only one possible way it uses URI but in this case, it is possible to have an error 414 (Request-URI Too Long).

I would like to return this feature back.
server/src/test/java/org/elasticsearch/action/fieldcaps/FieldCapabilitiesRequestTests.java
Consumer<FieldCapabilitiesRequest> mutator = randomFrom(mutators);
        mutator.accept(mutatedInstance);
        return mutatedInstance;
    }
    public void testToXContent() throws IOException {
        FieldCapabilitiesRequest request = new FieldCapabilitiesRequest();
        request.indexFilter(QueryBuilders.termQuery("field", "value"));
        request.runtimeFields(singletonMap("day_of_week", singletonMap("type", "keyword")));
        XContentBuilder builder = XContentFactory.contentBuilder(XContentType.JSON);
        String xContent = BytesReference.bytes(request.toXContent(builder, ToXContent.EMPTY_PARAMS)).utf8ToString();
        assertEquals(("""
            {  "index_filter": {
                "term": {
                  "field": {
                    "value": "value"
                  }
                }
              },
              "runtime_mappings": {
                "day_of_week": {
                  "type": "keyword"
                }
              }
            }""").replaceAll("\\s+", ""), xContent);
    }
    public void testValidation() {
        FieldCapabilitiesRequest request = new FieldCapabilitiesRequest().indices("index2");
        ActionRequestValidationException exception = request.validate();
        assertNotNull(exception);
    }