tris: advertise PSS in Client Hello for TLS 1.3

Until PSS support is finalized for TLS 1.2, let's not advertise support
for that. Since TLS 1.3 however mandates PSS, we have no other option
than advertising this even if we have not added complete support.

Another reason why I apply it to just TLS 1.3 and not 1.2 is because the
latter would require updating the testdata.

common.go

@@ -184,6 +184,22 @@ var supportedSignatureAlgorithms = []SignatureScheme{
 	ECDSAWithSHA1,
 }
 
+// supportedSignatureAlgorithms13 lists the advertised signature algorithms
+// allowed for digital signatures. It includes TLS 1.2 + PSS.
+var supportedSignatureAlgorithms13 = []SignatureScheme{
+	PSSWithSHA256,
+	PKCS1WithSHA256,
+	ECDSAWithP256AndSHA256,
+	PSSWithSHA384,
+	PKCS1WithSHA384,
+	ECDSAWithP384AndSHA384,
+	PSSWithSHA512,
+	PKCS1WithSHA512,
+	ECDSAWithP521AndSHA512,
+	PKCS1WithSHA1,
+	ECDSAWithSHA1,
+}
+
 // ConnectionState records basic TLS details about the connection.
 type ConnectionState struct {
 	ConnectionID                []byte                // Random unique connection id

handshake_client.go

@@ -105,6 +105,7 @@ NextCipherSuite:
 		// set legacy_version to TLS 1.2 for backwards compatibility.
 		hello.vers = VersionTLS12
 		hello.supportedVersions = config.getSupportedVersions()
+		hello.supportedSignatureAlgorithms = supportedSignatureAlgorithms13
 	}
 
 	return hello, nil