This change adds realm name of the realm used to perform authentication to the responses of _security/oidc/authenticate and _security/oidc/authenticate APIs

Resolves elastic#53161

Author: BigPandaToo

x-pack/qa/oidc-op-tests/src/test/java/org/elasticsearch/xpack/security/authc/oidc/OpenIdConnectAuthIT.java

@@ -420,7 +420,6 @@ private Tuple<String, String> completeAuthentication(String redirectUri, String
             logger.info(" OpenIDConnect authentication response {}", responseBody);
             assertNotNull(responseBody.get("access_token"));
             assertNotNull(responseBody.get("refresh_token"));
-            assertNotNull(responseBody.get("realm"));
             assertNotNull(responseBody.get("authentication"));
             assertEquals("alice", ((Map)responseBody.get("authentication")).get("username"));
             return Tuple.tuple(responseBody.get("access_token").toString(), responseBody.get("refresh_token").toString());
@@ -518,13 +517,11 @@ class PrepareAuthResponse {
         private URI authUri;
         private String state;
         private String nonce;
-        private String realm;
 
-        PrepareAuthResponse(URI authUri, String state, String nonce, String realm) {
+        PrepareAuthResponse(URI authUri, String state, String nonce, @Nullable String realm) {
             this.authUri = authUri;
             this.state = state;
             this.nonce = nonce;
-            this.realm = realm;
         }
 
         URI getAuthUri() {
@@ -539,7 +536,5 @@ String getNonce() {
             return nonce;
         }
 
-        String getRealm() { return realm; }
-
     }
 }