Add support for SSH connection using openSSH certificates in SSHHook

[oWnKeyKing]

Using SSH certificates is good way to secure and manage ssh connections.
It allows storing short-lived keys and easily revoking them in case of compromise.

The extra field in the ssh_conn object is not limited and it is possible to add keys and values as needed. In this case, it would be convenient to add an ssh_certificate field.

Unfortunately, the SSHOperator and SSHHook do not currently support connection management via this method.

To improve the Operator, the hook should be able to establish a connexion by ssh certificate when the field "ssh_certificate" is provided.

If I'm not mistaken, SSHHook uses the paramiko lib, which has the load_certificate() function which could fit to the solution