Ability to pass client certificate to backend

**What would you like to be added**:
The ability to pass client certificates to the backend when using frontendValidation within the Gateway resource.
The path is: spec.listeners.tls.frontendValidation.
When this is enabled, the frontend prompts the client to provide their certificate, which should then be passed to the backend services in a header.

**Why this is needed**:
In a lot of scenarios, some applications needs a client certificate. This is a standard procedure, a lot of other ingress controllers has.

Start with spec.listeners.tls.frontendValidation in the Gateway resource and read the linked GEP overview before proposing a change. Define how the client certificate is represented in a backend header and what Gateway API behavior should be specified; done means the enhancement has an agreed design and corresponding API specification.

Written by the indexing model from the issue text.