Add federation component for Cargohold

[pcapriotti]

This PR simply adds Cargohold as a new federated component in Federator, and a federated API stub in Cargohold itself.

Note: this contains the first few commits from #1966.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If HTTP endpoint paths have been added or renamed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.

[jschaul]

CI seems to not have been able to install things for this PR (I retriggered CI); but the changeset looks good.

[jschaul]

This block also needs to be added to the similar place under charts/federator/ for CI to pass.