[FEAT] Add support for webauthn credential level 3

- For WebAuthn Credential level 3, the `backup_eligible` and
`backup_state` flags are checked if they are consistent with the values
given on login. Forgejo never stored this data, so add a database
migration that makes all webauthn credentials 'legacy' and on the next
first use capture the values of `backup_eligible` and `backup_state`.
As suggested in https://github.com/go-webauthn/webauthn/discussions/219#discussioncomment-10429662
- Adds unit tests.
- Add E2E test.

models/forgejo_migrations/v22.go

@@ -0,0 +1,17 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package forgejo_migrations //nolint:revive
+
+import "xorm.io/xorm"
+
+func AddLegacyToWebAuthnCredential(x *xorm.Engine) error {
+	type WebauthnCredential struct {
+		ID             int64 `xorm:"pk autoincr"`
+		BackupEligible bool  `xorm:"NOT NULL DEFAULT false"`
+		BackupState    bool  `xorm:"NOT NULL DEFAULT false"`
+		Legacy         bool  `xorm:"NOT NULL DEFAULT true"`
+	}
+
+	return x.Sync(&WebauthnCredential{})
+}