Add `Gitlab::SSHPublicKey.supported_types` method.
This method returns the names of ssh keys GitLab supports.
`Gitlab::SSHPublicKey` should be the source of truth about such
information.
`SUPPORTED_KEY_TYPES` has the same information so I reused the method
to prevent duplication that would allow easier to extend the list of
supported keys.
See https://gitlab.com/gitlab-org/gitlab/-/issues/213259.