Improve Gateway API features

[cfi2017]

• In order to support TLS Re-encryption with Gateway API the chart should support deploying a BackendTLSPolicy for the service.
• In order to support TLS passthrough (eg. for security for client cert authentication) the chart should offer an example TLSRoute configuration in the default values.yaml file
• For advanced use-cases, once it's stabilised it'd be good to support ListenerSets.

I'd be happy to work on this once I have some free time.

[rissson]

Hello, we would indeed accept PRs for all of those.