Support host certificates in gitlab-sshd
## Summary We need to add config support for the `host_cert_files` parameter for `gitlab-sshd`. This was added in https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/661. `host_cert_files` contains a list of signed certificates for the server's host keys. This is equivalent to OpenSSH's `HostCertificate` parameter. The idea here is that some certificate authority signs off on the server's keys, and `gitlab-sshd` publishes these certificates so clients don't have to rely on manual fingerprint checking. There will likely be one certificate per host key used. We'll probably want to pass the content of the public keys in as a Kubernetes secret, and then save them to a mount that `gitlab-sshd` can access.
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