Support for mTLS (browser to load balancer)

[timeyr]

It would be great to have support for mTLS between the browser and the load balancer. This is supported on the application load balancer in general, it just cannot be configured right now using the k8s Gateway API.

There are two pieces that would be important:

1. Being able to attach a ServerTlsPolicy (containing the trust config holding the client root CA) to the TargetHttpsProxy managed by the Gateway Controller
2. Being able to configure mTLS custom headers on the UrlMap managed by the Gateway Controller