Make durable client registration idempotent.

[jviau]

This PR addresses an issue where registering the durable worker extension twice would break the extension. The extension registration has now been made idempotent and safe to call multiple times. Additionally, durable extension registration has been made available in an extension method off IFunctionsWorkerApplicationBuilder to simplify cases where auto-registration via source gen is not available.

Issue describing the changes in this PR

resolves #2804

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.

[cgillum]

@jviau these changes look good to me. Can you comment on whether this has been tested? If so, I'd feel good about merging it.

[jviau]

@cgillum I have ran smoke tests and it succeeds. Also succeeds with a redundant call to ConfigureDurableExtension (added in this PR) to confirm it addresses the double-registration issue.

[MarcinJuraszek]

Has this been released in any version already?

[cgillum]

@MarcinJuraszek yes, see the release notes here: https://github.com/Azure/azure-functions-durable-extension/releases/tag/v1.2.0-worker-extension