Create BBM to migrate intrinsic artifacts:expose_as data from ci_builds_metadata/ci_job_definitions into p_ci_job_artifacts

Context

options => artifacts:expose_as is needed long term for the MR widgets exposing artifacts and it's used when has_exposed_artfacts: true

This should be considered intrinsic data because it's used in MR widgets to display artifacts.

With #549079 (closed) we will have started writing intrinsic artifacts data to the columns exposed_as and exposed_paths in p_ci_job_artifacts. Also the logic to read from either the old or new data sources should be in place.

Proposal

In this issue we now migrate the existing intrinsic artifacts data from ci_build_metadata (or ci_job_definitions, depending on the timing) to p_ci_job_artifacts.

After this is migration is complete, we can update the code to only read from the new source.

Note: We should consider running pg_repack on p_ci_job_artifacts after the migration. See the usage stats of "exposed artifacts" here: #550223 (comment 2604368410).