better dashapi testing

[tarasmadan]

Is your feature request related to a problem? Please describe.
I'd like to test dashapi with gcs/spanner dependencies.
The problematic API is a gcsPayloadHandler.

Describe the solution you'd like
I'd like to mock the dependencies and get maximum flexibility.
We have mockery and spanner mocks.
GCS mocks could be added the same way.

The question is how to inject mocks into the tested dashboard.
We are widely using context.Context tree to propagate similar data.

The proposal is to propagate context into the mocking targets constructors.
Constructor will be able to return the mock(if present) instead of real object.