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

routers/api/v1/user/app.go

@@ -62,6 +62,8 @@ func ListAccessTokens(ctx *context.APIContext) {
 			Name:           tokens[i].Name,
 			TokenLastEight: tokens[i].TokenLastEight,
 			Scopes:         tokens[i].Scope.StringSlice(),
+			Created:        tokens[i].CreatedUnix.AsTime(),
+			Updated:        tokens[i].UpdatedUnix.AsTime(),
 		}
 	}