Check for existing executor

[jviau]

Check if an IFunctionExecutor has already been registered for the function invocation. If one already is registered, durable extension will skip registering its own executor.

Issue describing the changes in this PR

resolves microsoft/agent-framework#2327

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
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• I have added all required tests (Unit tests, E2E tests)
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  • 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
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  • 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.
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  • 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.