feat(http): add oauth2 refresh token client

packages/wyatt_http_client/lib/src/utils/header_keys.dart

@@ -14,17 +14,8 @@
 // You should have received a copy of the GNU General Public License
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
-enum HeaderKeys {
-  authorization('Authorization'),
-  wwwAuthenticate('WWW-Authenticate'),
-  contentType('Content-Type');
-
-  final String name;
-
-  @override
-  String toString() {
-    return name;
-  }
-
-  const HeaderKeys(this.name);
+abstract class HeaderKeys {
+  static const String authorization = 'Authorization';
+  static const String wwwAuthenticate = 'WWW-Authenticate';
+  static const String contentType = 'Content-Type';
 }