[Feature Request] TLS termination within server

In very strict security setup, we may want to ensure all traffic are encrypted in transit. Including communications between authgear and application or ingress controller.

We can enable k8s ingress controller with https backend:
https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/annotations/#backend-protocol

or SSL passthrough:
https://kubernetes.github.io/ingress-nginx/user-guide/tls/#ssl-passthrough

typical setup:
```
--https--> k8s-ingress --https--> authgear
--https--> k8s-ingress --https--> application --https--> authgear
--https--> k8s-ingress --https--> application ----https----> k8s-ingress --https--> authgear
```

Optionally, supports mTLS.

No repository file, test, or entry point is named. Start by locating the server's listener and TLS configuration, then review the referenced Kubernetes ingress backend-protocol and SSL passthrough documentation. Done should include encrypted server communication and a defined approach for the optional mTLS support, with tests covering the resulting behavior.

Written by the indexing model from the issue text.