[WIP] Re-add ScheduledStartTime to orchestration scheduling in .NET isolated

[davidmrdavid]

Follow up to: https://github.com/Azure/azure-functions-durable-extension/pull/2634 https://github.com/Azure/azure-functions-durable-extension/pull/2544#issuecomment-1688579724

Fixes: https://github.com/microsoft/durabletask-dotnet/issues/256

When refactoring the localGrpcListener (used by .NET isolated and Java) to use an IDurableClient (necessary for Distributed Tracing), we accidentally dropped the ability to set an orchestrator's "fireAt" time. This PR adds that back.

Since the IDurableClient does not expose functionality for scheduled orchestrators, this PR constructs the TaskHubClient, which does expose that functionality. It also leaves a few "TODO" comments to simplify the implementation in the future.    Sitll pending:   Manual E2E test   Inclusion of distributed tracing regression test  * Inclusion of test of scheduled orchestrations test.  

Issue describing the changes in this PR

resolves https://github.com/microsoft/durabletask-dotnet/issues/256

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 v3.x branch.
  • [ ] Otherwise: This change only applies to Durable Functions v2.x and will not be merged to branch v3.x.

[developmentvegas]

Hi, Is there any update on this? Several users are stuck trying to use the startAt and we can't ;(

[davidmrdavid]

This is missing some tests still unfortunately. Looking to get back to this soon, but have some urgent competing threads at the moment. If anyone is willing to contribute some tests in the meantime (described in the PR description), I'd be happy to incorporate them.