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

packages/backend/package.json

@@ -31,6 +31,7 @@
     "@backstage/plugin-scaffolder-backend": "^0.1.1-alpha.21",
     "@backstage/plugin-sentry-backend": "^0.1.1-alpha.21",
     "@backstage/plugin-techdocs-backend": "^0.1.1-alpha.21",
+    "@gitbeaker/node": "^23.5.0",
     "@octokit/rest": "^18.0.0",
     "dockerode": "^3.2.0",
     "example-app": "^0.1.1-alpha.21",