T215: Implement client side support for u2f keys (including tests)
Description
Originally reported by Jakuje: https://bugs.libssh.org/T215
The OpenSSH 8.2p1 implements support for keys backed by u2f enabled hardware inside of ssh protocol:
https://cvsweb.openbsd.org/src/usr.bin/ssh/PROTOCOL.u2f?annotate=HEAD
The internals support was added recently in 17b518a6 as well as the server support (key verification), but client support and tests are still missing. The testing can be done in very similar manner as in OpenSSH, which ships sk-dummy.so module, which can be used instead of physical hardware.