Re-introduce SSH key comments via the API

Summary

The issue #220185 (closed) would break the terraform provider: https://github.com/terraform-providers/terraform-provider-gitlab/issues/338

It is pairing the behaviour of an UNAUTHENTICATED endpoint (github.com/users.key) with an AUTHENTICATED one (deploy key API)

From a security perspective, it is probably not useful to hide ssh key comments, especially on authenticated api's.

/label regression