Make gwctl ReferenceGrant aware. Capture any errors observed while constructing the resource graph.

[gauravkghildiyal]

What type of PR is this?

/kind feature

What this PR does / why we need it:

• Use ReferenceGrants to determine if a cross-namespace reference from HTTPRoute to a Backend is valid.
• Capture incorrect reference errors (like reference not found or not permitted) in the resourceModel. This will be available for consumption by the views.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

gwctl now displays "incorrect" or "not-found" reference errors. It also uses ReferenceGrants to validate if a reference is correct or not (incorrect reference errors are made visible to the user)

/area gwctl

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gauravkghildiyal

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• ~~gwctl/OWNERS~~ [gauravkghildiyal]

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[gauravkghildiyal]

/cc @mlavacca

[gauravkghildiyal]

/retest

[robscott]

Thanks @gauravkghildiyal!

/lgtm

[gauravkghildiyal]

/retest