Mark group as optional in LocalObjectReference

[tamalsaha]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Make group as optional in LocalObjectReference

Which issue(s) this PR fixes:

Fixes #3322

Does this PR introduce a user-facing change?:

Mark group as optional in LocalObjectReference

[k8s-ci-robot]

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[youngnick]

/ok-to-test

[arkodg]

also need to make Group a pointer and add an omitempty tag

[tamalsaha]

Done!

[arkodg]

this may not be a breaking change for users, but may be a breaking change for implementations, I still +1 this change to improve user experience (lesser config to write)

[tamalsaha]

/retest

[robscott]

Let's hold off on this for a minute, I think this actually requires a bit more discussion.

/hold

Although I certainly would love to save some unnecessary config here if we can, I'm worried that we're going to end up with some unintended consequences.

As far as I can tell, there are currently 3 places in the API where we have a default group, and in all cases we also default kind:

1. ParentRefs have a default group of gateway.networking.k8s.io and a default kind of Gateway
2. BackendRefs have a default group "" and a default kind of Service
3. SecretRefs (CertRef and ClientCertRef) have a default group of "" and a default kind of Secret

It looks like we're currently using LocalObjectReference in 2 places:

1. ExtensionRef in HTTPRoute and GRPCRoute
2. CACertificateRefs in BackendTLSPolicy

It seems very clear that we'd never want to default to the core ("") API group for ExtensionRef. CACertificateRefs is a little bit less clear on the surface, but given that our goal is to move to recommending something like ClusterTrustBundle here when it's ready instead of ConfigMaps, I think we'd regret defaulting to "" here.

So with all that said, my proposed change here would be much smaller. We'd just change "When unspecified or empty string" to "When empty string".