ssh/agent: match OpenSSH extensionAgentMsg, not IETF draft

The OpenSSH wire format just suffixes the raw extension body,
without a nested string.

Fixes golang/go#51689

Change-Id: Ic224cedb934ba0563abca9a45a6be1c67769ed6d
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/412154
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Run-TryBot: hopehook <hopehook@qq.com>
Reviewed-by: Daniel Lublin <daniel@lublin.se>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

ssh/agent/client.go

@@ -226,7 +226,9 @@ var ErrExtensionUnsupported = errors.New("agent: extension unsupported")
 
 type extensionAgentMsg struct {
 	ExtensionType string `sshtype:"27"`
-	Contents      []byte
+	// NOTE: this matches OpenSSH's PROTOCOL.agent, not the IETF draft [PROTOCOL.agent],
+	// so that it matches what OpenSSH actually implements in the wild.
+	Contents []byte `ssh:"rest"`
 }
 
 // Key represents a protocol 2 public key as defined in