Copy the certificate secret from the origin namespace to the namespace of Istio gateway (#3724)
Copy the certificate secret from the origin namespace to the namespace of Istio gateway (#3724)

* initial version of reconciling secrets

* refactor copy cert logic to make it clear

* update copy cert

* make things work

* update the comment

* add unit tests

* delete the unused comment

* add one more test case to increase test coverage

* Update pkg/reconciler/v1alpha1/clusteringress/clusteringress_test.go

fix typo

Co-Authored-By: ZhiminXiang <zhiminx@google.com>

* address comment

* use UID as a part of name of target secret

* simply unit test

* Update pkg/reconciler/v1alpha1/clusteringress/resources/secret.go

Co-Authored-By: ZhiminXiang <zhiminx@google.com>

* fix the broken unit test

* address comment

* address comment

* add licence to secret_test.go

* fix the broken unit test

* address comment

* address comment

* fix broken unit test

knative/networking,knative/networking