Python: Consolidate workflow ID instantiation

[TaoChenOSU]

Currently, workflow can get ID from two places:

1. Randomly generated at build time
2. From a checkpoint

The Id is also managed by 3 entities during runtime:

1. The workflow object itself
2. The runner
3. The runner context

We are currently doing nothing to make sure the ID in these 3 places stay in sync.

We should consider a way to have a non-random identifier for workflow. Currently, the traces from different workflow instances are separated because their IDs are different. This creates an issue when you have a persisted session, which creates different instances of workflows but following the same checkpoint state lineage. In this case, there isn't a way to "join" these separate traces from different instances.

[lokitoth]

We should take care to avoid confusing Workflow Id with Run Id (and Checkpoint Id) (as well as WorkflowExecutor Id when hosted in another workflow).

And we should make sure that .NET carries the same semantics.