This adds support for specifying host certificates via the
`host_cert_files` option and advertises the signed key to the
client. This acts similarly to OpenSSH's `HostCertificate` parameter:
gitlab-sshd attempts to match a host key to its certificate, and then
substitutes the matching host key with a certificate signed by a
trusted certificate authority's key.

This is the first requirement to supporting SSH certificates. This
will enable the client to trust the server if both trust a common
certificate authority. The `TrustedUserCAKeys` option will need to be
supported later for the server to trust all user keys signed by this
certificate authority.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/495

The config option is basically a copy of LoginGraceTime OpenSSH
option.

If an SSH connection is hanging unauthenticated, after some period
of time, the connection gets canceled. The value is configurable,
the server waits for 60 seconds by default.

It seems that a lot of users rely on this, let's return it and
deprecated later to make the migration less disruptive

We don't support diffie-hellman-group14-sha1 via OpenSSH currently
Let's avoid introducing it in gitlab-sshd because it's using
weak hashing algorithm

Issue: https://gitlab.com/gitlab-org/gitlab-shell/-/issues/576

ProxyHeaderTimeout must be small to avoid DoS risk

Let's make the value configurable and 500ms by default

MACs, Ciphers and KEX algorithms now can be configured
If the values are empty, reasonable defaults are used

Git clients sometimes open a connection and leave it idling,
like when compressing objects.
Settings like timeout client in HAProxy might cause these
idle connections to be terminated.

Let's send the keepalive message in order to prevent a client
from closing

It would give us more flexibility when we decide to enable
PROXY protocol