feat: return time of last usage for public keys and access tokens in the api (#34323)

In the Gitea GUI, the user can see the time that _AccessTokens_ and
_PublicKeys_ were last used. This information is not returned by the
_/users/{username}/tokens_ and _/user/keys_ endpoints in the API. This
PR adds the missing data.

The time of last usage for for _tokens_ & _keys_ seem to be stored in
the _Updated_ field of the structs internally. For consistency, I have
used the name _updated_at_ for the new field returned by the _API_.
However, for the _API_ user, I don't think that name reflects the data
returned, as I believe it is the time of last usage. I propose that we
use the name _last_used_at_ instead. Let's hear reviewers opinion on
that.

* PublicKey
  1. _last_used_at_: string($date-time)
* AccessToken
  1. _created_at_: string($date-time) (for parity with public keys)
  2. _last_used_at_: string($date-time)

Fix #34313

services/convert/convert.go

@@ -316,6 +316,7 @@ func ToPublicKey(apiLink string, key *asymkey_model.PublicKey) *api.PublicKey {
 		Title:       key.Name,
 		Fingerprint: key.Fingerprint,
 		Created:     key.CreatedUnix.AsTime(),
+		Updated:     key.UpdatedUnix.AsTime(),
 	}
 }