Add option to accept new fingerprint

[clue]

Right now, this project will complain when you're connecting to a server for the first time and you have not already accepted have its fingerprint. You may simply connect on the command line with ssh user@example.com and accept this initial fingerprint (TOFU: trust on first use).

We should add an option to pass an explicit fingerprint to this library so that one no longer has to connect to a server on the command line before being able to use this library.

[clue]

See also the provisional ssh URI scheme definition in https://www.iana.org/assignments/uri-schemes/prov/ssh:

ssh://[<user>[;fingerprint=<host-key fingerprint>]@]<host>[:<port>]

See also expired draft https://datatracker.ietf.org/doc/draft-salowey-secsh-uri/ & https://www.ietf.org/archive/id/draft-salowey-secsh-uri-00.html