Add support for a certificate file to be passed to `asyncssh.connect()`

[Andrew-S-Rosen]

What should we add?

This is essentially a mimic of AgnostiqHQ/covalent-slurm-plugin#45, and the PR there (AgnostiqHQ/covalent-slurm-plugin#47) would work equally well here.

Currently, client_keys in asyncssh.connect() is set to client_keys=[self.ssh_key_file], and self.ssh_key_file is coded such that it can only be a single path, as shown below. However, the authentication process on some machines requires a list[tuple(str, str)] to be passed that has the SSH key and a certificate file, respectively.

covalent-ssh-plugin/covalent_ssh_plugin/ssh.py

Lines 224 to 229 in dcd5fc2

	conn = await asyncssh.connect(
	self.hostname,
	username=self.username,
	client_keys=[self.ssh_key_file],
	known_hosts=None,
	)

Describe alternatives you've considered.

There are no alternatives if you require a certificate to SSH because the current code is hard-coded to only accept an SSH key as the authentication method.