Updating Kafka CloudEvent source

[nachocano]

Fixes knative/eventing#1205

Proposed Changes

• Changing Kafka CloudEvent source to include more information than just the topic.
• Changed it from <topic> to /apis/v1/namespaces/<namespace>/kafkasources/<source_name>#<topic>

Release Note

Updating Kafka CloudEvent source. This is a breaking change in the sense that if Triggers have been created listening for events on a particular Kafka topic, they will have to update their source field inside SourceAndType filter to listen to /apis/v1/namespaces/<namespace>/kafkasources/<source_name>#<topic>

[nachocano]

/assign @matzew

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
contrib/kafka/pkg/adapter/adapter.go	14.5%	41.2%	26.7
contrib/kafka/pkg/apis/sources/v1alpha1/kafka_types.go	100.0%	93.3%	-6.7
contrib/kafka/pkg/reconciler/kafkasource.go	86.0%	89.2%	3.2

[matzew]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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