Allow Channel$ kinds and messaging API version in Subscriptions

[nachocano]

Helps with #1216

Proposed Changes

• Allow to create Subscriptions where their spec.channel does not just allow the Channel kind (^Channel$), but also other kind of channels.
• The change allows any kind that ends in Channel (Channel$).
• Also added support for the apiVersion messaging.knative.dev/v1alpha1.
• This is needed for the different channels CRDs. E.g., InMemoryChannel, KafkaChannel, NATSSChannel, etc.

Release Note

Subscriptions can now have different Channel kinds as part of their spec.channel. E.g., "InMemoryChannel", "KafkaChannel", etc, and support the "messaging.knative.dev/v1alpha1` apiVersion.

[nachocano]

/assign @Harwayne

If approved, this should be added as part of the Channel Spec. That is, the naming convention for Channel kinds should end in Channel.

[Harwayne]

The code looks fine, but I'm not sure we want to require the name to end in Channel. For now, my preference is anything matching the Subscribable duck type is allowed, regardless of name. However, I don't have a good idea how to actually implement that...

[nachocano]

The code looks fine, but I'm not sure we want to require the name to end in Channel. For now, my preference is anything matching the Subscribable duck type is allowed, regardless of name. However, I don't have a good idea how to actually implement that...

Makes sense. Let me see if I can find a way to do that. I'll put a TODO in the code in case this blocks us, and we can solve it later on.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: nachocano
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[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/eventing/v1alpha1/subscription_validation.go	100.0%	97.0%	-3.0

[nachocano]

Just talked with @evankanderson and @akashrv and we agreed that we will remove the validation for ^Channel$, we will let the Subscription be created, and if it's not subscribable, then we will mark it as failed in the controller... Closing this PR