Implement [email protected] extension

[gnodet]

Description

See https://github.com/openssh/openssh-portable/blob/V_9_8/PROTOCOL#L167-L184

This protocol extension allows the SSH2_MSG_EXT_INFO to be sent
during user authentication. RFC8308 does allow a second
SSH2_MSG_EXT_INFO notification, but it may only be sent at the end
of user authentication and this is too late to signal per-user
server signature algorithms.

Support for receiving the SSH2_MSG_EXT_INFO message during user
authentication is signalled by the client including a
"[email protected]" key via its initial SSH2_MSG_EXT_INFO
set after the SSH2_MSG_NEWKEYS message.

A server that supports this extension MAY send a second
SSH2_MSG_EXT_INFO message any time after the client's first
SSH2_MSG_USERAUTH_REQUEST, regardless of whether it succeed or fails.
The client SHOULD be prepared to update the server-sig-algs that
it received during an earlier SSH2_MSG_EXT_INFO with the later one.

Motivation

Even without implementing per-user server signature algorithms, supporting this extension on the server side would bring better support when connecting to OpenSSH servers.

Alternatives considered

No response

Additional context

No response