Implementing SSH_MSG_EXT_INFO (RFC8308)

[devtekve]

Adds support for https://datatracker.ietf.org/doc/html/rfc8308 (server side only though)

When support for RFC8332 was added, the point 3.3 was probably skipped since it's a SHOULD .

Discovery of Public Key Algorithms Supported by Servers

Implementation experience has shown that there are servers that apply
authentication penalties to clients attempting public key algorithms
that the SSH server does not support.

Servers that accept rsa-sha2-* signatures for client authentication
SHOULD implement the extension negotiation mechanism defined in
[RFC8308], including especially the "server-sig-algs" extension.

When authenticating with an RSA key against a server that does not
implement the "server-sig-algs" extension, clients MAY default to an
"ssh-rsa" signature to avoid authentication penalties. When the new
rsa-sha2-* algorithms have been sufficiently widely adopted to
warrant disabling "ssh-rsa", clients MAY default to one of the new
algorithms.

This PR addresses that and RSA keys are working as expected. This resolves #49