Add KEDA support for GCP Sources

[nachocano]

Fixes #423

Proposed Changes

• Using annotations to configure autoscaling, as done in serving. The annotation names might change.
• If you don't specify the keda autoscaling class (currently the only one supported), then the sources will be treated as non-scalable.
• Separated reconciler for Pullsubscription into two. A keda-based one, which understands about KEDA ScaledObject and checks whether KEDA is installed... And a k8s one, which reconciles a plain deployment.
• Adding a Resource duck type to be able to track changes to KEDA ScaledObjects. Should be upstreamed to pkg. Also leveraging ListableTracker from eventing, should probably upstream it to pkg as well.
• Using Unstructured for ScaledObject. As KEDA still doesn't have client-go support for its types, and advises to use the dynamic client, I consider unnecessary to add the dependency for now. Also, it will bring a bunch of things like controller-runtime, etc.

TODOs:

• Docs (in another PR)
• E2E tests (in another PR)

Release Note

CloudPubSubSource, CloudSchedulerSource, CloudStorageSource, and CloudAuditLogsSource now support Keda-based scheduling. They can scale all the way down to zero. Note that you will need to have Keda installed. Refer to the installation docs for more details.

[nachocano]

/assign @grantr
/assign @capri-xiyue

[nachocano]

/assign @ericlem

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[ericlem]

/assign @ericlem

[nachocano]

/assign @ericlem

[nachocano]

/retest

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/apis/duck/v1alpha1/annotations.go	Do not exist	93.2%
pkg/apis/duck/v1alpha1/register.go	0.0%	100.0%	100.0
pkg/apis/duck/v1alpha1/resource_types.go	Do not exist	25.0%
pkg/apis/events/v1alpha1/cloudpubsubsource_validation.go	92.3%	88.9%	-3.4
pkg/apis/events/v1alpha1/cloudschedulersource_validation.go	92.9%	93.1%	0.2
pkg/apis/events/v1alpha1/cloudstoragesource_defaults.go	75.0%	60.0%	-15.0
pkg/apis/events/v1alpha1/cloudstoragesource_validation.go	93.3%	93.5%	0.2
pkg/apis/events/v1alpha1/register.go	0.0%	100.0%	100.0
pkg/apis/pubsub/v1alpha1/pullsubscription_types.go	100.0%	90.9%	-9.1
pkg/apis/pubsub/v1alpha1/pullsubscription_validation.go	90.3%	87.5%	-2.8
pkg/reconciler/pubsub/pullsubscription/keda/controller.go	Do not exist	95.5%
pkg/reconciler/pubsub/pullsubscription/keda/pullsubscription.go	Do not exist	63.4%
pkg/reconciler/pubsub/pullsubscription/keda/resources/names.go	Do not exist	100.0%
pkg/reconciler/pubsub/pullsubscription/keda/resources/scaled_object.go	Do not exist	100.0%
pkg/reconciler/pubsub/pullsubscription/static/controller.go	Do not exist	95.2%
pkg/reconciler/pubsub/pullsubscription/static/pullsubscription.go	Do not exist	72.7%
pkg/reconciler/pubsub/reconciler.go	71.0%	71.3%	0.3
pkg/reconciler/pubsub/resources/annotations.go	Do not exist	100.0%
pkg/reconciler/pubsub/resources/pullsubscription.go	87.5%	85.7%	-1.8

[knative-prow-robot]

@nachocano: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-google-knative-gcp-go-coverage	010eb44	link	/test pull-google-knative-gcp-go-coverage

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[capri-xiyue]

/lgtm

[capri-xiyue]

/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: capri-xiyue, nachocano

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