Implement the client credentials grant

crates/storage-pg/migrations/20230904135550_oauth2_client_credentials_grant.sql

@@ -13,4 +13,9 @@
 -- limitations under the License.
 
 -- This makes the user_id in the oauth2_sessions nullable, which allows us to create user-less sessions
-ALTER TABLE oauth2_sessions ALTER COLUMN user_id DROP NOT NULL;
+ALTER TABLE oauth2_sessions
+    ALTER COLUMN user_id DROP NOT NULL;
+
+-- This adds a column to the oauth2_clients to allow them to use the client_credentials flow
+ALTER TABLE oauth2_clients
+    ADD COLUMN grant_type_client_credentials boolean NOT NULL DEFAULT false;