Generate Contour CRDs with V1 CustomResourceDefinition.

[jpeach]

Generating CRDs as v1 CustomResourceDefinition resources enables better CRD validation and defaulting, as well as the ability to use kubectl explain.

This requires Kubernetes 1.16 or later. We can do this as the default YAML, but we can also generate different sets of YAML (as demonstrated in #2474).

This issue tracks the decision for adding this, and we can also use it to collect other issues that are blocked on the CRD support.

[skriss]

A few notes here:

• personally, I don't think we need to generate both v1 and v1beta1 CRD YAML -- Kubernetes 1.16 (where CRDs moved to GA) has been out for a year, and folks can always pull an older release of Contour if they really need pre-1.16 support. Moving to generating just v1 CRDs is a lot simpler. Would like to get agreement from other maintainers on this, though.
• currently, our CRDs have spec.preserveUnknownFields set to true, because they're created as v1beta1 CRDs and that's the default value for that field in that version. We need to set that to false (which is the v1 default) because (a) we want the behavior of pruning unknown fields, and (b) it needs to be false to make kubectl explain work, apparently. Unfortunately, controller-gen doesn't seem to allow you to explicitly set it to false when generating v1 CRD YAML, so we'll need users to manually run a kubectl patch command to remove that field as part of the upgrade process. It won't be an issue on a fresh install of Contour using v1 CRD YAML, though, since false is the default.

Otherwise, this looks to be a pretty simple change that will add a lot of value going forward.

[youngnick]

I'm fine with only outputting the CRD v1 objects, as you say, I think 1.16 has been out long enough, and this functionality will save all our users a lot of headaches with simple formatting and validation errors.

I think having a kubectl patch command that will ensure that the preserveUnknownFields is set to false on everything is acceptable, we'll need to have that in the upgrade instructions for a release or two though.

[skriss]

As an update, I was able to add logic to our wrapper CRD generation script to add spec.preserveUnknownFields: false to our CRD YAML, so users won't have to do the manual patch.