Add support for ssh public key authentication

[PlqnK]

Hi!
First of all, thanks for this application, it's simple, beautiful and get the job done! And it being fully FOSS is really the icing on the cake!

But I don't really see a way to configure ssh key authentication in the app or talked about in the README.
A lot of people, like myself, just disable ssh password authentication entirely on their servers and not having ssh key support in your app make it unusable for us.

Would it be possible to add support for it? I've read rapidly the documentation of the ssh flutter package your application is using and it supports ssh key authentication using standard PEM encoded RSA keys.

There could be multiple implementation options like managing it directly in the app (like ConnectBot) or by storing the key file encrypted on the internal shared storage and have your app "just" read it for example, the password field could be used to input the private key decryption password.

Anyway, thanks again for your work, it's really appreciated!

[bungabunga]

I could open a separate issue about this feature request but I will write it here because it's connected:

Please when adding a support for SSH public key authentication consider also adding support for hardware tokens (such as Yubikey) as private key storages. There is an open source SDK you can use to add this feature: https://github.com/cotechde/hwsecurity

Thank you for developing this crucial android app as an open source software!