access: sftp: add public key auth options

Attempt public key authentication using user specified path.
Or, if user specified key file path unset.
Attempt public key authentication by trying common default file paths:
 ~/.ssh/id_rsa
 ~/.ssh/id_ed25519
 ~/.ssh/id_dsa
 ~/.ssh/id_ecdsa

In both cases fall back to password autentication if key authetication
fails.