feat(vm_ssh): support custom identity-file in 'vm ssh' command

[squizzi]

Add -i, --identity-file support to the vm ssh command to allow a user to specify a custom identity file when using replicated vm ssh.

Testing

# I get a password because it's not the right key
./bin/replicated vm ssh -i ~/.ssh/a-key-i-know-wont-work
✔ friendly_kalam (running)
The authenticity of host '[111.222.333.444]:42187 ([111.222.333.444]:42187)' can't be established.
ED25519 key fingerprint is SHA256:cyQAAVxafABejTt5GGt94nTChXLPiC23kFEzLQBIl6s.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '[111.222.333.444]:42187' (ED25519) to the list of known hosts.
[email protected]'s password:


# My actual key lets me in
/bin/replicated vm ssh -i ~/.ssh/id_ed25519
✔ friendly_kalam (running)
$ 

./bin/replicated vm ssh -i ~/not-real
Error: identity file does not exist or is not accessible: stat /Users/squizzi/not-real: no such file or directory