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
public void testDLS() throws IOException {
        Request createIndex = new Request("PUT", "/dls");
        createIndex.setJsonEntity("""
            {
                "mappings" : {
                    "runtime" : {
                        "year" : {
                          "type" : "keyword",\s
                          "script" : "emit(doc['date'].value.substring(0,4))"
                        }
                    },
                    "properties" : {
                        "date" : {"type" : "keyword"}
                    }
                }
            }
            """);
        assertOK(adminClient().performRequest(createIndex));
        Request indexDoc1 = new Request("PUT", "/dls/_doc/1");
        indexDoc1.setJsonEntity("""
            {
                "date" : "2009-11-15T14:12:12"
            }
            """);
        assertOK(adminClient().performRequest(indexDoc1));
        Request indexDoc2 = new Request("PUT", "/dls/_doc/2");
        indexDoc2.setJsonEntity("""
            {
                "date" : "2016-11-15T14:12:12"
            }