Open gRPC server for Python if requested

[andystaples]

Adds a new environment variable that allows changing the worker communication protocol to gRPC manually. Will be used for preview builds of the durabletask-azurefunctions package before support is added to Python worker + Host to allow WebJobs extensions to access worker capabilities

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation PR is ready to merge and referenced in pending_docs.md
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)
• My changes do not require any extra work to be leveraged by OutOfProc SDKs
  • Otherwise: That work is being tracked here: #issue_or_pr_in_each_sdk
• My changes do not change the version of the WebJobs.Extensions.DurableTask package
  • Otherwise: major or minor version updates are reflected in /src/Worker.Extensions.DurableTask/AssemblyInfo.cs
• My changes do not add EventIds to our EventSource logs
  • Otherwise: Ensure the EventIds are within the supported range in our existing Windows infrastructure. You may validate this with a deployed app's telemetry. You may also extend the range by completing a PR such as this one.
• My changes should be added to v2.x branch.
  • Otherwise: This change applies exclusively to WebJobs.Extensions.DurableTask v3.x. It will be retained only in the dev and main branches and will not be merged into the v2.x branch.