[9.18] new: ci: Allow pushing branches and tags to customer git repos

[9.18] new: ci: Allow pushing branches and tags to customer git repos

For pipelines in the private repository, add an optional manual job,
which allows the current branch to be pushed into the specified
customer's git repository. This can be useful to provide patch previews
for early testing.

For tags created in a private repository, add a manual job which pushes
the created tag to all entitled customers.

Backport of MR !10323

Merge branch 'backport-nicki/ci-customer-git-automation-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10357