adding e2e test PubSubSource->Broker with PubSubChannel

[nlopezgi]

First part of #511

Proposed Changes

• Add an e2e test that does CloudPubSubSource -> Broker backed by PubSub channel

Refactored some of the existing code for the test that uses Broker backed by PubSub channel with generic Source.
Left a couple of TODOs about refactoring and validations, need a bit of advice for those.

This PR is still a draft, my first attempt to create an e2e test, so not sure this is the right direction yet.

fyi @nachocano

[nlopezgi]

fyi @chizhg

[nachocano]

@nlopezgi just as fyi, by adding WIP in the description then is considered a draft and cannot be merged, until you un-WIP it..

Took a quick look, and yes, I agree, the less duplication we have the better. So I'd suggest you go forward with a refactor.
/assign @chizhg for suggestions on refactoring?

[chizhg]

/cc

[chizhg]

/lgtm
/assign @nachocano

[nachocano]

/approve

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