introduce multi-project pipelines

Motivation:

One of the marketplace web-server dependencies is
the marketplace-webapp.

Hence any changes in this repository should trigger
a new ci/cd in the marketplace web-server.

Modification:

- separate the test phase from the build.
- add a trigger to the deploy stage

Result:

Each new merge request and commit will create a new
ci/cd build in the marketplace web-server repository.

Target: master
Review: https://gitlab.hzdr.de/hifis-technical-platform/helmholtz-marketplace-webapp/-/merge_requests/4
8 jobs for master in 10 minutes and 58 seconds (queued for 18 seconds)