2nd attempt at https://github.com/elastic/elasticsearch/issues/84591

references https://github.com/elastic/elasticsearch/issues/48856#issuecomment-831144741

tldr; the permission structure for the endpoint isn't useful for most users. If users can create runtime fields they should have access to this api, they shouldn't need a specific permission set. 
x-pack/qa/runtime-fields/with-security/src/javaRestTest/java/org/elasticsearch/xpack/security/PermissionsIT.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; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.security;
import org.elasticsearch.action.fieldcaps.FieldCapabilitiesRequest;
import org.elasticsearch.action.fieldcaps.FieldCapabilitiesResponse;
import org.elasticsearch.action.search.SearchRequest;
import org.elasticsearch.action.search.SearchResponse;
import org.elasticsearch.client.Request;
import org.elasticsearch.client.RequestOptions;
import org.elasticsearch.client.RestClient;
import org.elasticsearch.client.RestHighLevelClient;
import org.elasticsearch.common.document.DocumentField;
import org.elasticsearch.common.settings.SecureString;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.concurrent.ThreadContext;
import org.elasticsearch.search.SearchHit;
import org.elasticsearch.search.builder.SearchSourceBuilder;
import org.elasticsearch.test.rest.ESRestTestCase;
import org.junit.AfterClass;
import org.junit.Before;
import java.io.IOException;
import java.util.Collections;
import java.util.Map;
@SuppressWarnings("removal")