[UserAPI] [UserIndicator Plugin] [ExampleUserProvider Plugin] New user api and related functionality (#4538)

* Implements User API and example user plugin

Co-authored-by: John Hill <john.c.hill@nasa.gov>
Co-authored-by: Charles Hacskaylo <charlesh88@gmail.com>
Co-authored-by: Andrew Henry <akhenry@gmail.com>

src/api/user/constants.js

@@ -0,0 +1,24 @@
+/*****************************************************************************
+ * Open MCT, Copyright (c) 2014-2021, United States Government
+ * as represented by the Administrator of the National Aeronautics and Space
+ * Administration. All rights reserved.
+ *
+ * Open MCT is licensed under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ * Open MCT includes source code licensed under additional open source
+ * licenses. See the Open Source Licenses file (LICENSES.md) included with
+ * this source code distribution or the Licensing information page available
+ * at runtime from the About dialog for additional information.
+ *****************************************************************************/
+
+export const MULTIPLE_PROVIDER_ERROR = 'Only one user provider may be set at a time.';
+export const NO_PROVIDER_ERROR = 'No user provider has been set.';