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 class PermissionsIT extends ESRestTestCase {
    private static HighLevelClient highLevelClient;
    private static HighLevelClient adminHighLevelClient;
    @Override
    protected Settings restClientSettings() {
        String token = basicAuthHeaderValue("test", new SecureString("x-pack-test-password".toCharArray()));
        return Settings.builder().put(ThreadContext.PREFIX + ".Authorization", token).build();
    }
    @Override
    protected Settings restAdminSettings() {
        String token = basicAuthHeaderValue("test_admin", new SecureString("x-pack-test-password".toCharArray()));
        return Settings.builder().put(ThreadContext.PREFIX + ".Authorization", token).build();
    }
    @Before
    public void initHighLevelClient() {
        if (highLevelClient == null) {
            highLevelClient = new HighLevelClient(client());
            adminHighLevelClient = new HighLevelClient(adminClient());
        }
    }
    @AfterClass
    public static void closeHighLevelClients() throws IOException {
        highLevelClient.close();
        adminHighLevelClient.close();
        highLevelClient = null;
        adminHighLevelClient = null;
    }