Add CEL for BackendTLSPolicy targetRefs

[snorwin]

What type of PR is this?

/kind feature

What this PR does / why we need it:
This PR introduces validation to ensure that targetRefs on BackendTLSPolicy are unique. Without this validation, implementations may face challenges in properly resolving these references. A CEL is introduced, similar to the one used in the HTTPRoute, to facilitate this validation.

Which issue(s) this PR fixes:

None

Does this PR introduce a user-facing change?:

CEL validation for target references in BackendTLSPolicy

[k8s-ci-robot]

Hi @snorwin. Thanks for your PR.

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[robscott]

This is a great addition, thanks for the work on this @snorwin!

/cc @candita

[robscott]

/ok-to-test

[robscott]

Thanks @snorwin! Will defer final LGTM to @candita

/approve

[candita]

@snorwin It would be great if you could attend the next community meeting and do a small presentation on the background and solution, for the awareness of other implementers. Also, a godoc update would be super helpful to all.

[snorwin]

@snorwin It would be great if you could attend the next community meeting and do a small presentation on the background and solution, for the awareness of other implementers. Also, a godoc update would be super helpful to all.

@candita I added a description similar to the one for the parentRefs in HTTPRoute, but I've simplified it a bit for clarity.

I'll do my best to attend the community meeting, but the pre-xmas season is quite busy for me.

[mlavacca]

Thanks, @snorwin!

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mlavacca, robscott, snorwin

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[candita]

@snorwin It would be great if you could attend the next community meeting and do a small presentation on the background and solution, for the awareness of other implementers. Also, a godoc update would be super helpful to all.

@candita I added a description similar to the one for the parentRefs in HTTPRoute, but I've simplified it a bit for clarity.

I'll do my best to attend the community meeting, but the pre-xmas season is quite busy for me.

@snorwin I asked in the Slack channel. I would prefer to make this more well-known now that we have 5 implementations and are aiming to move BackendTLSPolicy to standard.

[candita]

/lgtm

Hold until this is more widely known and accepted.
/hold

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[robscott]

Thanks for the work on this @snorwin! I've added this to the agenda for our next meeting (Jan 6), and one of us can summarize the changes then (no worries if you can't make it).

I think this is a strict improvement and unlikely to be problematic to implementations, but agree with @candita that we should make sure this is widely known in advance of the v1.3 release. Since this is on the agenda now for the next meeting, I'm fine with merging this at any point, but will defer to @candita on the timing.