Added support for Entity Trigger

[davidmrdavid]

The Durable Entities construct of Durable Functions uses a trigger binding type named entityTrigger. This PR makes a minimal change that enables entityTrigger to enable Durable Functions for Python to define Entities. This is will enable the following PR for Durable Functions in Python: Azure/azure-functions-durable-python#184

[Hazhzeng]

Please import this EntityContext in azure/functions/__init__.py
https://github.com/Azure/azure-functions-python-library/blob/dev/azure/functions/__init__.py#L14

[vrdmr]

@davidmrdavid - Please add tests for the new type in here - test_durable_functions.py. Also, when this gets done, also add to the E2E tests in worker - the second piece will come in later but just FYI.

[vrdmr]

@davidmrdavid Any updates on this?

[davidmrdavid]

@vrdmr , I haven't gotten around this yet because I was OOF last week and got a bit sidetracked with some other tasks today. I want to make this a priority for tomorrow though. I will be pinging you for a few clarifications on this in the morning, sorry for the delay! Just for my information, is this blocking a release?

[davidmrdavid]

@Hazhzeng @vrdmr ,
Made the requested changes (exporting the EntityContext) and generalized the existing unit tests to account for the EntityContext as well.

@Hazhzeng , I also left a TODO on the existing unit tests as I realized that these two tests are identical. Please let me know if I'm missing something there.

[vrdmr]

Thanks for the changes - Let's have a quick session on how the EntityTrigger is used.