t.conch.ssh.SSHServerTransport should have a separate list for server-sig-algs

[adiroiban]

The current code is here

twisted/src/twisted/conch/ssh/transport.py

Lines 1715 to 1718 in 63df84e

	if firstKey:
	self.sendExtInfo(
	[(b"server-sig-algs", b",".join(self.supportedPublicKeys))]
	)

server-sig-algs is documented in RFC 8308

This extension is sent by the server and contains a list of public
key algorithms that the server is able to process as part of a
"publickey" authentication request.

The current list is shared between server host keys algorithms and userauth pubkey algorithms.

You might want to have a SSH server that for example only supports ssh-ed25519 and ecdsa-sha2-nistp256 as a host keys... but users should still be allowed to authenticate with an ecdsa-sha2-nistp521 or ssh-rsa key

---

One option to fix this is to have a separate list of algorithms allowed for userauth pubkey that is sent to server-sig-algs

It can be named SSHServerTransport.supportedSigAlgs