Expose last mirror sync timestamp through the API

[petergardfjall]

Feature Description

One can queue up a sync operation for a mirror repo via POST /api/v1/{owner}/{repo}/mirror-sync.
However, there is no way for an API client of learning of completion (the UI does expose this information in the Last update mirror setting field, but not the REST API).

This issue proposes to add a mirror_updated field to the repository type (as returned by GET /api/v1/{owner}/{repo}).
For example, this allows API clients to determine if an initiated sync has completed, or take decisions on if a mirror repo should be synced (given some client-side policy on freshness).

[petergardfjall]

PR in #18267