fs/ssh: allow prompting for password/passphrase when necessary

[guludo]

Currently, the option ask_password for SSh remotes only allows values true and false. This is a proposal of a third value: if_nedeed, which makes dvc prompt for password/passphrase if necessary. This is useful for cases when the user is using a private key for authentication: the private key passphrase would be prompted for only if it is encrypted.

For background on the discussion on Discord, it starts here: https://discord.com/channels/485586884165107732/485596304961962003/917572804734496840

Best regards,
Gustavo Sousa