Parse 'sub' to identify resource owner

As per https://tools.ietf.org/html/rfc7662#section-2.2 the `sub` key should
identify the resource owner in oauth2 introspection responses. 

This change adds support for the `sub` key and will allow the introspection 
response of RFC-compliant servers to be parsed.

Will still try `user_id` first as to not break backward compatibility.

openid-connect-client/src/main/java/org/mitre/oauth2/introspectingfilter/IntrospectingTokenService.java

@@ -244,7 +244,10 @@ public class IntrospectingTokenService implements ResourceServerTokenServices {
 	private Authentication createUserAuthentication(JsonObject token) {
 		JsonElement userId = token.get("user_id");
 		if(userId == null) {
-			return null;
+			userId = token.get("sub");
+			if (userId == null) {
+				return null;
+			}
 		}
 
 		return new PreAuthenticatedAuthenticationToken(userId.getAsString(), token, introspectionAuthorityGranter.getAuthorities(token));