ssh: support rsa-sha2-256/512 for client authentication

CL 220037 had implemented support for host authentication using
rsa-sha2-256/512, but not client public key authentication. OpenSSH
disabled the SHA-1 based ssh-rsa by default in version 8.8 (after
pre-announcing it in versions 8.2, 8.3, 8.4, 8.5, 8.6, and 8.7) although
some distributions re-enable it. GitHub will start rejecting ssh-rsa for
keys uploaded before November 2, 2021 on March 15, 2022.

https://github.blog/2021-09-01-improving-git-protocol-security-github/

The server side already worked, as long as the client selected one of
the SHA-2 algorithms, because the signature flowed freely to Verify.
There was however nothing verifying that the signature algorithm matched
the advertised one. The comment suggested the check was being performed,
but it got lost back in CL 86190043. Not a security issue because the
signature had to pass the callback's Verify method regardless, and both
values were checked to be acceptable.

Tested with OpenSSH 8.8 configured with "PubkeyAcceptedKeyTypes -ssh-rsa"
and no application-side changes.

The Signers returned by ssh/agent (when backed by an agent client)
didn't actually implement AlgorithmSigner but ParameterizedSigner, an
interface defined in an earlier version of CL 123955.

Updates golang/go#49269
Fixes golang/go#39885
For golang/go#49952

Change-Id: I13b41db8041f1112a70f106c55f077b904b12cb8
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/392394
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

ssh/common.go

@@ -297,8 +297,9 @@ func (c *Config) SetDefaults() {
 }
 
 // buildDataSignedForAuth returns the data that is signed in order to prove
-// possession of a private key. See RFC 4252, section 7.
-func buildDataSignedForAuth(sessionID []byte, req userAuthRequestMsg, algo, pubKey []byte) []byte {
+// possession of a private key. See RFC 4252, section 7. algo is the advertised
+// algorithm, and may be a certificate type.
+func buildDataSignedForAuth(sessionID []byte, req userAuthRequestMsg, algo string, pubKey []byte) []byte {
 	data := struct {
 		Session []byte
 		Type    byte
@@ -306,7 +307,7 @@ func buildDataSignedForAuth(sessionID []byte, req userAuthRequestMsg, algo, pubK
 		Service string
 		Method  string
 		Sign    bool
-		Algo    []byte
+		Algo    string
 		PubKey  []byte
 	}{
 		sessionID,