fix: native parsing of ssh certificate key (#6953)

- In the case of parsing an public SSH certificate key, use the underlying key type instead of the certificate type. This means `ed25519-cert-v01` would be seen as `ed25519` type and thus correctly parsed. Certificates do not
change the keysize or otherwise parsing of the key.
- Add unit test.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6953
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

models/asymkey/ssh_key_parse.go

@@ -219,8 +219,13 @@ func SSHNativeParsePublicKey(keyLine string) (string, int, error) {
 		return "", 0, fmt.Errorf("ParsePublicKey: %w", err)
 	}
 
+	pkeyType := pkey.Type()
+	if certPkey, ok := pkey.(*ssh.Certificate); ok {
+		pkeyType = certPkey.Key.Type()
+	}
+
 	// The ssh library can parse the key, so next we find out what key exactly we have.
-	switch pkey.Type() {
+	switch pkeyType {
 	case ssh.KeyAlgoDSA:
 		rawPub := struct {
 			Name       string