Expose kpt package rendering errors via the status of PackageVariant

[kispaljr]

Fixes nephio-project/nephio#730

This implementation exposes the render status of downstream package revisions in the downstreamTargets field of PackageVariant's status.

[kispaljr]

The reason behind exposing the RenderStatus as a Conditions is that it doesn't require to modify the CRD/schema days before Release date.

If CRD modifications are allowed, then I would consider adding a renderStatus field to items of status.downstreamTargets as a more elegant solution.

[kispaljr]

I rewrote the PR to expose render status via status.downstreamTargets. As I commented above I consider this a superior solution, even though it requires a minor change in the PC schema.

[kispaljr]

/retest

[liamfallon]

/approve
/lgtm

[liamfallon]

/approve
/lgtm

[nephio-prow]

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