### 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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.action.fieldcaps;
import org.elasticsearch.action.ActionRequestValidationException;
import org.elasticsearch.action.support.IndicesOptions;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.bytes.BytesReference;
import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
import org.elasticsearch.common.io.stream.Writeable;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.ArrayUtils;
import org.elasticsearch.index.query.QueryBuilders;
import org.elasticsearch.search.SearchModule;
import org.elasticsearch.test.AbstractWireSerializingTestCase;
import org.elasticsearch.xcontent.ToXContent;