sourcegraph: add initContainers support

[michaellzc]

Checklist

• Follow the manual testing process
• Update changelog

Test plan

[michaellzc]

Current dependencies on/for this PR:

• main
  • PR sourcegraph: add initContainers support #246 👈

This comment was auto-generated by Graphite.

[marcleblanc2]

Closing: this branch has been untouched since Feb 2023 and now conflicts with main.

• The motivating request (extraInitContainers needed #384, extraInitContainers for cloud-sql-proxy) was closed as won't-do, since init containers run sequentially and can't serve the migrator.
• The one init-container value that has landed since is grafana.initContainers (Add grafana.initContainers for Airgapped Analytics #941), added as a per-service value rather than this shared helper.

If generic <service>.initContainers support is still wanted, it should be a fresh PR off current main following the #941 pattern.