feat(datasource): add Gitlab Package support (#11672)

lib/constants/platform.spec.ts

lib/constants/platforms.ts

lib/datasource/api.ts

...tasource/gitlab-packages/__snapshots__/index.spec.ts.snap

lib/datasource/gitlab-packages/common.ts

lib/datasource/gitlab-packages/index.spec.ts

lib/datasource/gitlab-packages/index.ts

lib/datasource/gitlab-packages/readme.md

lib/datasource/gitlab-packages/types.ts