Add support for openkeychain's sshauthentication-api

[chekovma]

OpenKeychain provides PGP support for Android. Using it allows access to authentication keys stored on hardware tokens (yubikeys). The developers have created an API to allow accessing those keys for ssh auth: https://github.com/open-keychain/open-keychain/tree/master/sshauthentication-api adding support to Amaze would mean allowing secure 2FA for sftp (ssh) connections.

[chekovma]

Termbot: https://github.com/open-keychain/termbot is an example app (fork of connectbot) already implementing this api

[EmmanuelMess]

@TranceLove you might want to have a look at this one.

[TranceLove]

Blocked by https://github.com/open-keychain/open-keychain/issues/2452, until OpenKeychain allows simply import SSH key offline and ask user for the Yubikey when needed.

Alternate solutions are always welcomed :)