gep: refine ClientCertificateRef description for backend TLS

[snorwin]

What type of PR is this?

/kind gep

What this PR does / why we need it:
In the current version of the GEP-3155, the semantics of an invalid ClientCertificateRef are not fully specified.
Additionally, the GEP states that an invalid ClientCertificateRef affects the ResolvedRefs condition on the Listeners, which I think is incorrect.

This PR clarifies the behavior by introducing a ResolvedRefs condition on the Gateway instead. That condition will be set whenever a ClientCertificateRef is invalid or not permitted.

Which issue(s) this PR fixes:

N/A

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: snorwin
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[snorwin]

/cc @robscott @kl52752 @rikatz

[kl52752]

This statement is no longer true. gateway client certificate is part of gateway identity and should be the same for all connections from gateway to backend, this setting cannot be overridden on service level

[snorwin]

I was thinking the same, but I didn’t want to remove it without discussing it first. If everyone agrees, I’ll go ahead and remove it.

[rikatz]

/assign

[rikatz]

overall lgtm.

Will leave final word for approvers :)

[snorwin]

@robscott @youngnick what do you think about adding a ResolvedRefs condition to the Gateway? Would that make sense to you, or should we instead extend the Accepted condition with another reason? In general, how would an invalid client certificate affect acceptance?

I’m interested to hear your thoughts so we can improve the description further.