Save runner taggings to shard table

What does this MR do and why?

The runner's tags are saved in the taggings table and with this MR we're starting to duplicate that data into the new ci_runner_taggings table with the end goal of removing the taggings table.

References

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• Save taggings data to ci_runner_taggings (#472974 - closed)
• Save taggings data to ci_build_tags (!162276 - merged)

MR acceptance checklist

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Screenshots

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Feature.enable(:write_to_ci_runner_taggings)
runner = Ci::Runner.last
runner.tag_list = ['ruby', 'docker']
runner.save!

Testing that runner is synced to partitioned table before saving tags:

Comment out the call to ensure_partitioned_runner_record_exists and run the new tests at spec/models/ci/runner_spec.rb:96.