Add shared runner builds table to the database [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This merge request adds a new table used to track builds running on shared runners.

Once a new build gets picked by a shared runner, we insert a tracking entry to the ci_shared_runner_builds table. We remove the entry when a build transitions to a new state from running to any (using state machine).

See #329765 (closed) for more details.

Conformity

• I have included a changelog entry.
• I have added information for database reviewers in the MR description.
• I have self-reviewed this MR per code review guidelines.

Migrations

== 20210601123341 AddRunningBuildsTable: migrating ============================
-- create_table(:ci_running_builds, {})
   -> 0.1501s
== 20210601123341 AddRunningBuildsTable: migrated (0.1502s) ===================

Issues

Resolves #329765 (closed)