Refactor broker to have plugable way to reconcile a service

[yolocs]

As multiple people have suggested, this is an attempt to split #2425
The ksvc implementation will be contributed to eventing-contrib as well as the ksvc broker controller.

With the ongoing work for broker class, this change clears the way to enable the ksvc broker controller (to be contributed to eventing-contrib) with other eventing core controllers.

To be clear, this is just a local refactoring.

Release Note

[yolocs]

@vaikas this is slightly related with the broker class work because my plan is to fork the broker controller in eventing contrib.

[yolocs]

@vaikas @mattmoor slightly related with the ongoing work to split channel broker from core controller

[evankanderson]

/assign @vaikas @n3wscott

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[yolocs]

This PR has been updated to keep up with the recent broker reconciler changes. @n3wscott

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/apis/eventing/v1alpha1/broker_lifecycle.go	95.8%	88.5%	-7.4
pkg/reconciler/broker/broker.go	79.4%	75.3%	-4.1
pkg/reconciler/broker/resources/ingress.go	0.0%	100.0%	100.0
pkg/reconciler/internal/service/kube/kube.go	Do not exist	85.9%

[yolocs]

@lionelvillard this is a follow-up for splitting #2425