[Cassandra] Make GetWorkflowExecution() fetch RunID internally if not provided

[gow]

What changed?

GetWorkflowExecution() in Cassandra now fetches the current runID if one is not provided.

Why?

Making changes to persistence interface to keep storage specific implementation details behind persistence interface. This is the first of the many changes to come that will gradually move relevant storage specific logic from application to persistence interface.

How did you test it?

Added new unit tests

Potential risks

Keep an eye on Cassandra queries templateGetCurrentExecutionQuery & templateGetWorkflowExecutionQuery

Documentation

N/A

Is hotfix candidate?

No