Support running with specific runner tags

Description

Currently there is no out of the box way to configure jobs to run with a specific (tagged) runner.

This can still be accomplished by overwriting jobs, but this is kind of a hassle and not straightforward:

# Define the tags to use for all jobs
.python-base: &job_tags
  tags:
    - python
    - test
.python-test:
  <<: *job_tags

# Build jobs
py-package:
  <<: *job_tags
py-lint:
  <<: *job_tags
...

Maybe it is sufficient to just configure this for .python-base, but it should at least be documented nonetheless.

Implementation ideas

Maybe provide four tag options, one global one, and more specific tags for build, test and publish stages.