ssh: add support for SSH_AGENT_CONSTRAIN_EXTENSION with id 255

it was changed in the following draft

https://datatracker.ietf.org/doc/html/draft-miller-ssh-agent-03

The id 3 is now used for SSH_AGENT_CONSTRAIN_MAXSIGN key constraint,
an OpenSSH extension to the protocol that we do not currently support.
Instead, we added a compatibility layer for
SSH_AGENT_CONSTRAIN_EXTENSION with ID 3.

Fixes golang/go#62311

Change-Id: I421aee92aee9e693e43f66e6a5515c055333cb9b
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/525355
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

ssh/agent/server_test.go

@@ -243,7 +243,11 @@ func TestParseConstraints(t *testing.T) {
 			ExtensionDetails: []byte(fmt.Sprintf("details: %d", i)),
 		}
 		expect = append(expect, ext)
-		data = append(data, agentConstrainExtension)
+		if i%2 == 0 {
+			data = append(data, agentConstrainExtension)
+		} else {
+			data = append(data, agentConstrainExtensionV00)
+		}
 		data = append(data, ssh.Marshal(ext)...)
 	}
 	_, _, extensions, err := parseConstraints(data)