Allow for using custom SSH keys, including encrypted keys

[bortels]

I get why - but it might be nice to at least have an option to choose and use a pre-existing encrypted ssh key, or to encrypt the key and save the password for it in the keychain (I'm on macOS).

[toby]

I agree! You should be able to specify the key you'd like to use for your account.

[toby]

You can now use a custom key with the CHARM_IDENTITY_KEY env var:

https://github.com/charmbracelet/charm/blob/main/client/client.go#L36

We should probably also provide a global -i flag and we also still need to support encrypted keys. I'll open up two new issues for that though.

[toby]

Added #107 and #106.