SSH agent forwarding

[msimkunas]

The "multipass shell" command currently does not support SSH agent forwarding.

It's always possible to connect to the vm via the ssh client manually but being able to use the "multipass shell" command to connect to the vm with SSH agent forwarding would be a significant DX improvement.

[xmkg]

Related to: #3137

[ricab]

From man ssh_config:

Agent forwarding should be enabled with caution. Users with the ability to bypass file permissions
on the remote host (for the agent's Unix-domain socket) can access the local agent through the for‐
warded connection. An attacker cannot obtain key material from the agent, however they can perform
operations on the keys that enable them to authenticate using the identities loaded into the agent.

I'd be more inclined to automatically importing the user's public key or even let them ssh <instance_name> directly and then let them manage it. There's #1957 for that, with lots more comments and suggestions in #913.