Defer loading keys until they're needed

[goblin]

It would be nice if, when using SSHClient with callbacks/awaitables provided for the passphrase:

1. the callback asking for a passphrase to an unencrypted key wasn't ever called,
2. the callbacks asking for a passphrase for keys that won't be used when connecting to a host that doesn't need them or rejects them weren't called,
3. (perhaps) the passphrase callback would be called again up to 3x (or a configurable number) if the passphrase wasn't correct.

This might require loading the .pub files which contain public keys and presenting them to the server to check if they get accepted.

I understand this is a rather big change, and might require some deeper changes. It's not urgent at all for me, the new callback/awaitable support for passphrases is really great. Leaving this in case you'd like to address this in the future, as discussed on #426.

[ronf]

Thanks for filing this! I'll keep this in mind in future work on SSH key objects.