Add durable functions support

[Hazhzeng]

Background

We want to enable durable functions in Azure Functions Python Worker.
Previously, we are blocked by the implicit return from the generic binding.
The solution in this PR is to define a rich binding for OrchestrationTrigger and allow implicit returns from the trigger.

Depends On

Azure/azure-functions-python-library#48
CI is failing due to dependencies on azure-functions-python-library

Resolves

resolves: #608
resolves: #227

Usage

// function.json
{
  "scriptFile": "__init__.py",
  "bindings": [
    {
      "name": "orctx",
      "type": "orchestrationTrigger",
      "direction": "in"
    }
  ],
  "disabled": false
}

# main.py
def main(orctx: func.OrchestrationContext):
    orchestrate = df.Orchestrator.create(generator_function)
    result = orchestrate(orctx.body)
    return result

[scgbear]

The failing tests due to a dependence on the python-library changes?

[Hazhzeng]

@scgbear Yes, this is due to dependency on azure-functions-python-library

[asedighi]

i verified the code with @Hazhzeng and tested the faninfanout sample with durable + retry. I confirmed that it worked.