add support for rsa-sha2 signatures

[ploxiln]

Add support for rsa-sha2-256 and rsa-sha2-512, as well as the ext-info / server-sig-algs in handshake, used to advertise support for these new signature types.

I don't want to just copy exactly how it was handled in upstream paramiko, because if it doesn't get the ext-info message it assumes the new signature types for rsa keys are supported, rather than the inverse.

I've stalled out some months ago, because I've run into some buggyness ... and this is actually pretty tricky and messy to implement in this codebase, and my motivation ran out 🤷

References I'm working from:

• initial commit upstream paramiko@363a28d
• agent support paramiko@2b66625
• first fixup paramiko@d2865ed
• test paramiko@69fb31f
• fix logging/exceptions for server-sig-algs paramiko@6699d35
• fix key certs with new rsa sigs paramiko@4ef50df
• fix old cryptography lib compat paramiko@43189ef
• fix host key certs paramiko@7a2c84a
• fix rsa cert compat with openssh < 7.8 paramiko@dd1a32c + Fix OpenSSH<7.8 + RSA-CERT paramiko/paramiko#2077
• kex algorithms list issue with unusual client/server impls kexinit - use same kex algorithms as in security_options paramiko/paramiko#2083