[IMP] webservice_server_env: expose new OAuth2 options to server env

The webservice module gained new OAuth2 configuration fields (the token
request method, the client authentication method, and the custom
Authorization header name and value). These are now also manageable
through server environment configuration files, like the other
webservice fields.

Author: ivantodorovich

webservice_server_env/models/webservice_backend.py

@@ -29,6 +29,10 @@ def _server_env_fields(self):
             "oauth2_authorization_url": {},
             "oauth2_token_url": {},
             "oauth2_audience": {},
+            "oauth2_token_method": {},
+            "oauth2_client_auth_method": {},
+            "oauth2_client_auth_header": {},
+            "oauth2_client_auth_value": {},
         }
         webservice_fields.update(base_fields)
         return webservice_fields