Add Durable Entities support

[andystaples]

Add support for durable entities to this SDK

Also fixes a related bug: When the orchestration yields with CompleteOrchestration, we were discarding any pending actions - we need to keep these around for "side effects" actions like unlocking or signaling entities, which the user may expect to complete when the orchestration finishes.
Another issue with the side effects was discovered in the DTS backend, PR is linked in the test comments and the relevant test will need to be re-enabled once the fix is merged.

See the included documentation/samples/tests for usage guidelines.

Reviewers: Request special attention to file structures, naming, docs and tests

Discussion points:

• Optional parameters to some of the entity requests, like start time and cancellation tokens - should these be implemented now or later?
• Typing for entity calls - look at TInput/TOutput for activities, compare with entities (abstraction because of EntityInstanceId)
• Class-based entities - should we have the object state represent the entity state, or preserve the get_state & from_state behavior here (research into the dotnet SDK required)

[andystaples]

Will clean this up

[berndverst]

I'll merge - you can clean it up in a follow up PR.

[andystaples]

I can do this with a callback, where we just pass the _exit_critical_section method insetad of the whole OrchestrationContext, I just worry if it abstracts things a little too much. Feedback welcome