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
SearchRequest searchRequest = new SearchRequest("fls").source(new SearchSourceBuilder().docValueField("hidden_values_count"));
        {
            SearchResponse searchResponse = adminHighLevelClient.search(searchRequest, RequestOptions.DEFAULT);
            assertEquals(3, searchResponse.getHits().getTotalHits().value);
            for (SearchHit hit : searchResponse.getHits().getHits()) {
                assertEquals(1, hit.getFields().size());
                assertEquals(1, (int) hit.getFields().get("hidden_values_count").getValue());
            }
        }
        {
            SearchResponse searchResponse = highLevelClient.search(searchRequest, RequestOptions.DEFAULT);
            assertEquals(3, searchResponse.getHits().getTotalHits().value);
            for (SearchHit hit : searchResponse.getHits().getHits()) {
                assertEquals(0, (int) hit.getFields().get("hidden_values_count").getValue());
            }
        }
    }
    public void testFLSOnRuntimeField() throws IOException {
        Request createIndex = new Request("PUT", "/fls");
        createIndex.setJsonEntity("""
            {
                "mappings" : {
                    "runtime" : {
                        "year" : {
                          "type" : "keyword",\s
                          "script" : "emit(doc['date'].value.substring(0,4))"
                        }
                    },
                    "properties" : {
                        "date" : {"type" : "keyword"}