Make Subscribable type and its subtypes convert from/to v1/v1alpha1

[aliok]

Problem
KafkaChannel for example is being bumped from v1alpha1 to v1beta1. But the Subscribable API it uses jumps from v1alpha1 to v1.
I needed to hardcode some stuff in my PR: knative/eventing-contrib#1371

We should better implement this conversion support.

Additional context (optional)
See slack chat: https://knative.slack.com/archives/C9JP909F0/p1593764728273200

Plan:

• Make things implement apis.Convertible first. That'a a good structure. Handling conversion to all versions in 1 method. Otherwise, we'd end up creating multiple methods like "ConvertToV1", "ConvertToV1Beta1".
• Then implement conversion of v1alpha1<>v1
• Baby steps, multiple PRs for easy review. I did the whole conversion for all types in my local, the PR was super complicated as the types, signatures and the implementation was changed. Decided to create small PRs instead.

[aliok]

/assign

[aliok]

/close

All refactoring and clean up PRs are merged

[knative-prow-robot]

@aliok: Closing this issue.

Details

In response to this:

/close

All refactoring and clean up PRs are merged

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.