Remember the --auth=passwordless option when authenticating

[rosskusler]

What would you like Teleport to do? When someone authenticates to teleport using passwordless, tsh should remember this mode so that when the user needs to re-authenticate they'll be prompted for passwordless authentication instead of being prompted for a password. Maybe it could put an "auth: passwordless" entry into ~/.tsh/$clustername.yaml.

What problem does this solve? When I initially authenticate to teleport using "tsh login --mode=passwordless --proxy=REDACTED" it works successfully. But the next day (after my credentials have expired) when I do something with tsh/kubectl I get queried for my password. This fails because I dont have a password and I end up having to manually rerun "tsh login --auth=passwordless". It would be nice if tsh could just remember my auth mode from last time and prompt me for my MFA token.

If a workaround exists, please include it: It is possible to export TELEPORT_AUTH=passwordless

[codingllama]

Somewhat related: #13979.