Use K_GCP_AUTH_TYPE to differentiate authentication mode

[grac3gao-zz]

Fixes #

Proposed Changes

• First of the three steps to add the authentication check

1. Environment variable to differentiate authentication mode (<-this one)
2. Authentication check running inside the Pod
3. Additional k8s Event check for Secret absence

• Functionality includes:

1. Differentiate auth mode and pass K_GCP_AUTH_TYPE into underlying pods
2. Error out auth issue if required secret/k8s service account doesn't exist
3. Customized Event handler added for pullsubscription/topic/brokercell, so that once missing secret/k8s service account are added back, resources can immediately reconcile.

Release Note

- 🎁  Use K_GCP_AUTH_TYPE to differentiate authentication mode (first step for adding authentication check for source and broker)

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grac3gao

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[grac3gao-zz]

Ready to review. Addressed comments and added more UT for event handler

[grac3gao-zz]

/test pull-google-knative-gcp-wi-tests

[Harwayne]

I agree with @AlexandraRoatis that we should use the same keyword as the rest of the code, but my personal preference is to do it as a string literal. My reasoning is that if a test fails, it makes it very easy to grep/ctrl-F to find the test case.

Not needed for this PR, but I just don't want this to become the new style :)

[grac3gao-zz]

This makes sense, I agree that having a string to search when test fail is helpful for the debugging purpose (which I was not aware of before), and I guess this might be one of the reasons why we have names for each sub-UT. I'll change it.

[AlexandraRoatis]

Ok. I see the value in having the searchable string. I have no objection to reverting the changes.

[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/intevents/v1alpha1/brokercell_lifecycle.go	79.5%	92.5%	13.0
pkg/reconciler/brokercell/brokercell.go	94.7%	95.0%	0.4
pkg/reconciler/brokercell/controller.go	56.8%	57.8%	1.0
pkg/reconciler/events/build/controller.go	81.8%	83.3%	1.5
pkg/reconciler/intevents/pullsubscription/keda/controller.go	88.5%	88.9%	0.4
pkg/reconciler/intevents/pullsubscription/static/controller.go	88.0%	88.5%	0.5
pkg/reconciler/intevents/topic/controller.go	84.2%	85.0%	0.8
pkg/reconciler/intevents/topic/topic.go	72.0%	70.7%	-1.3
pkg/utils/authcheck/authtype.go	Do not exist	85.7%
pkg/utils/authcheck/enqueue.go	Do not exist	79.3%

[Harwayne]

/lgtm
/hold

Holding in case @AlexandraRoatis has any more comments.

[AlexandraRoatis]

/lgtm