Add GitLab integration for scaffolder

This adds a GitLab integration for the scaffolder backend.

We're introduceing a preparer and a publisher for GitLab so that we can
read templates from GitLab and publish them to a configured GitLab
instance. The two instances don't need to be the same. For instance,
templates could be public on gitlab.com, but the created repos will live
in a hosted GitLab somewhere else.

The publisher gets its own config object in `app-config.yaml` where the
target instance and token can be specified.

The service catalogue defines both `gitlab` and `gitlab/api` as
processors. They are both handled by the same preparer.

Closes #2372

app-config.yaml

@@ -116,6 +116,12 @@ scaffolder:
       $secret:
         env: GITHUB_ACCESS_TOKEN
     visibility: public # or 'internal' or 'private'
+  gitlab:
+    api:
+      baseUrl: https://gitlab.com
+      token:
+        $secret:
+          env: GITLAB_ACCESS_TOKEN
 
 auth:
   providers: