HTTPS Monitor: Client certificate authentication

Greetings,

Coming from uptime-kuma, there's something that makes this alternative currently unusable for my needs:

**Is your feature request related to a problem? Please describe.**
mTLS in UK allows to omit the CA field to allow client certificate authentication instead of true mTLS

**Describe the solution you'd like**
- Either make the CA optional in mTLS by leaving the field empty just like in UK

- Either add a Client cert auth select choice in the Authentication Method

Look for HTTPS monitor configuration and TLS client authentication code. The feature likely involves modifying the monitor's TLS setup to allow an empty CA field or adding a new authentication method. Check existing authentication handlers and TLS configuration structs. Test by setting up a monitor with client certificate authentication.

Rédigé par le modèle d'indexation à partir du texte de l'issue.