Allow calling sub-orchestrator by name

[andystaples]

Resolves #54

[Copilot]

Pull Request Overview

This PR adds support for calling sub-orchestrators by name (as a string) in addition to the existing support for passing function references. This brings consistency with the call_activity method which already supports both patterns.

• Updated the call_sub_orchestrator method signature to accept Union[Orchestrator[TInput, TOutput], str]
• Modified the implementation to handle both string names and function references
• Added comprehensive end-to-end tests for the new functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
durabletask/task.py	Updated abstract method signature to accept string orchestrator names
durabletask/worker.py	Implemented logic to handle both string and function reference orchestrators
tests/durabletask/test_orchestration_e2e.py	Added test validating sub-orchestrator calls by name
tests/durabletask-azuremanaged/test_dts_orchestration_e2e.py	Added test validating sub-orchestrator calls by name for Azure-managed scenarios

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