Add a manual job to trigger package build in omnibus

Merged Balasankar C requested to merge manual-build-trigger into master

What does this MR do?

Create a manual job in the pipeline that can be used to trigger a package build in omnibus-gitlab. These packages can be used by developers to test their changes.

Are there points in the code the reviewer needs to double check?

Make sure trigger happens and the variables are passed correctly.

What are the relevant issue numbers?

Closes: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2234

Related: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2250