feat: Add WebAuthn Cache
- Add IWebAuthnChallengeCacheProvider with distributed cache implementation for storing WebAuthn Challenges
- Inject the cache provider into AssertWebAuthnLoginCredentialCommand and WebAuthnGrantValidator so challenges can be stored
- Use a static token lifetime for WebAuthnLoginAssertionOptionsTokenable and enable nullable reference types on the tokenable
- Add unit tests for the cache provider, the assertion command, and the WebAuthn controller; add Identity integration and unit tests for WebAuthnGrantValidator with a FakeWebAuthnAuthenticator helper
