Proposal: Support Entity Locking

We need to support the Durable Functions `lock` API that allows users to establish a critical section for serial Entity updates. I believe this would be the first OOProc PL to support this feature, so I want to make sure we're in the same page about how we plan to implement it.

I imagine the API would be `yield`able, giving us an opportunity to pause the orchestrator execution if the lock cannot be readily acquired, and that it would take as argument a list of EntityIds.

In other words, I expect its interface to be as follows:

```Python
def lock(entityIds: List[string]):
```

### Proposed syntax and usage patterns

The first, and recommended, usage pattern would be one where the lock disposal is automatically managed by some context manager, so as follows:

```Python
entityIds: List[string] = ["myEntity1", "myEntity2", ... ]
with (yield context.lock(entityIds)):
# entity ops ... the lock is released at the end of this segment
```

The second usage pattern is one where entities are locked and released _manually_ by separate APIs
```Python
entityIds: List[string] = ["myEntity1", "myEntity2", ... ]
yield context.lock(entityIds)
yield context.release(entityIds)
```

I expect we'll want to support this second option because not all OOProc PLs with necessarily have a convenient lock-disposal syntax such as Python's "with" statement.

Please let me know if y'all have any thoughts. Otherwise, I'll proceed to prioritize this for an upcoming release. Thanks!