Fetches the pending post migrations artifact

What does this MR do and why?

Fetches the pending post migrations artifact via API by:

1. Fetch the pipelines triggered by the release-tools-bot on the release/tools project
2. Search for the build-post-deploy-migrations-artifact job
3. Once the job has been found download the artifact.

Previous approach was unreliable given it's not accurate to search a build by name nor to rely on the status of it.

This commit also renames the PostDeployMigrations::List class to Artifact to make it more descriptive

Related to gitlab-com/gl-infra/delivery#2362 (closed)

Author Check-list

• [-] Has documentation been updated?