Handle OpenID claims parameter in auth requests on Android (MaikuB#292)

The claims parameter is an optional part of the OpenID standard when making an authorization request (see https://openid.net/specs/openid-connect-core-1_0.html#ClaimsParameter).

Currently, this parameter is not supported by the Flutter interface layer, so the only way to pass it is via the additionalParameters argument when constructing an AuthorizationRequest. This, however, leads to the following error on Android:

```
E/AndroidRuntime( 7453): java.lang.IllegalArgumentException: Parameter claims is directly supported via the authorization request builder, use the builder method instead
```

This commit allows specifying the claims via additionalParameters and having this be correctly handled on Android, in order to be compatible with the `AuthorizationRequest.Builder` method.

flutter_appauth/android/src/main/java/io/crossingthestreams/flutterappauth/FlutterAppauthPlugin.java

@@ -22,6 +22,9 @@
 import net.openid.appauth.TokenResponse;
 import net.openid.appauth.connectivity.DefaultConnectionBuilder;
 
+import org.json.JSONException;
+import org.json.JSONObject;
+
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Map;
@@ -51,6 +54,7 @@ public class FlutterAppauthPlugin implements FlutterPlugin, MethodCallHandler, P
     private static final String TOKEN_ERROR_CODE = "token_failed";
     private static final String END_SESSION_ERROR_CODE = "end_session_failed";
     private static final String NULL_INTENT_ERROR_CODE = "null_intent";
+    private static final String INVALID_CLAIMS_ERROR_CODE = "invalid_claims";
 
     private static final String DISCOVERY_ERROR_MESSAGE_FORMAT = "Error retrieving discovery document: [error: %s, description: %s]";
     private static final String TOKEN_ERROR_MESSAGE_FORMAT = "Failed to get token: [error: %s, description: %s]";
@@ -333,6 +337,18 @@ private void performAuthorization(AuthorizationServiceConfiguration serviceConfi
                 authRequestBuilder.setUiLocales(additionalParameters.get("ui_locales"));
                 additionalParameters.remove("ui_locales");
             }
+
+            if(additionalParameters.containsKey("claims")){
+                try {
+                    final JSONObject claimsAsJson = new JSONObject(additionalParameters.get("claims"));
+                    authRequestBuilder.setClaims(claimsAsJson);
+                    additionalParameters.remove("claims");
+                }
+                catch (JSONException ex) {
+                    finishWithError(INVALID_CLAIMS_ERROR_CODE, ex.getLocalizedMessage(), getCauseFromException(ex));
+                    return;
+                }
+            }
             authRequestBuilder.setAdditionalParameters(additionalParameters);
         }