Add siphon-tables image bundling Siphon CDC table definitions [19.4]

What does this MR do and why?

Backports Add siphon-tables image bundling Siphon CDC tab... (!2960 - merged) to 19-4-stable. It adds the gitlab-siphon-tables image, which holds the Siphon CDC table definitions the Siphon chart uses to generate its CDC configuration.

Siphon is rolling out to GitLab Dedicated, which currently runs 19.3, so we backport to 19.3 and 19.4 only. The tables image built in Rails CI doesn't meet Dedicated's needs:

• Signing: it isn't signed with cosign. CNG signs all its images (Decide how Siphon table definitions are deliver... (gitlab-org/analytics-section/siphon#342)).
• Internal releases: Rails CI only builds it for tags, so tag-less internal releases get no image (https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/work_items/14420+).

The change only adds a new job and image. The only cherry-pick conflict was docs/build-process.md, which doesn't exist on this branch.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
  • This is effectively a maintenancepipelines or featureenhancement MR, which we normally don’t backport. However, because of its impact on Dedicated, we’re proposing this backport anyway.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).

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