KafkaSource and binding v1beta1<>v1alpha1 conversion

[aliok]

Part of #1332

Proposed Changes

• KafkaSource v1beta1<>v1alpha1 conversion test and conversion
• Same thing for KafkaBinding
• KafkaBinding and KafkaSource conversations share code

[mattmoor]

Produced via:
gofmt -s -w $(find -path './vendor' -prune -o -path './third_party' -prune -o -name '*.pb.go' -prune -o -type f -name '*.go' -print)
goimports -w $(find -name '*.go' | grep -v vendor | grep -v third_party | grep -v .pb.go | grep -v wire_gen.go)

[mattmoor]

Produced via:
gofmt -s -w $(find -path './vendor' -prune -o -path './third_party' -prune -o -name '*.pb.go' -prune -o -type f -name '*.go' -print)
goimports -w $(find -name '*.go' | grep -v vendor | grep -v third_party | grep -v .pb.go | grep -v wire_gen.go)

[aliok]

/unhold

[matzew]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aliok, matzew

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Needs approval from an approver in each of these files:

• OWNERS [matzew]

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[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
kafka/source/pkg/apis/bindings/v1alpha1/kafka_conversion.go	Do not exist	86.7%
kafka/source/pkg/apis/bindings/v1beta1/kafka_conversion.go	Do not exist	50.0%
kafka/source/pkg/apis/sources/v1alpha1/kafka_conversion.go	0.0%	94.4%	94.4