Implement Passkey registration

- Create Identity credentials
- Add management UI and registration flow

app/models/identity/credential.rb

@@ -0,0 +1,13 @@
+class Identity::Credential < ApplicationRecord
+  belongs_to :identity
+
+  serialize :transports, coder: JSON, type: Array, default: []
+
+  def to_public_key_credential
+    ActionPack::WebAuthn::PublicKeyCredential.new(
+      id: credential_id,
+      public_key: public_key,
+      transports: transports
+    )
+  end
+end