ref(server): Switches common endpoint to use `Managed`

[Dav1dde]

One step closer to replacing ManagedEnvelope with Managed<Envelope>.

This is also a first step into improving our outcome handling in endpoints (#5288) by providing a more explicit error handling. It remains to be seen how far we can use Rejected for these purposes though.

Active rate limit handling has been made a bit more explicit, it was already handled in a few different ways in endpoints, this is an attempt to make it a bit more explicit. It is necessary because we require common::handle_envelope to return Rejected<E>, which makes the active rate limits explicitly returned on success. Callers then have to decide what to do.

We may want to revisit the rate limiting behaviour of endpoints in the future again.