Ability to pass client certificate to backend

[Exobitt]

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.

[howardjohn]

Are you talking about XFCC or something else?

[Exobitt]

Are you talking about XFCC or something else?

Exactly that, yes.
In Traefik its X-Forwarded-Tls-Client-Cert.
Apparently, Envoy is X-Forwarded-Client-Cert

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