feat: add functionality to register plugins via http

Also:

- Add functionality for serving the responses from plugins
- Add tests for related methods and routes

Closes #2, #37

src/common.ts

@@ -13,7 +13,7 @@
  * You should have received a copy of the GNU Affero General Public License
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
-import {SCConfigFile} from '@openstapps/core';
+import {SCConfigFile, SCPluginMetaData} from '@openstapps/core';
 import {Validator} from '@openstapps/core-tools/lib/validate';
 import * as config from 'config';
 import {BackendTransport} from './notification/backend-transport';
@@ -37,3 +37,8 @@ export const validator = new Validator();
  * Provides information if the backend is executed in the "test" (non-production) environment
  */
 export const isTestEnvironment = process.env.NODE_ENV !== 'production';
+
+/*
+ * Stores a ("key-value") list of plugins where key is route and value is plugin information
+ */
+export const plugins = new Map<string, SCPluginMetaData>();