feat: add register_converter API

[hallvictoria]

The azure-functions-durable v2.x SDK uses the durabletask gRPC programming model, which requires orchestration and entity trigger outputs to be encoded as string datums rather than json. Previously there was no public way to override a registered converter.

This change introduces a minimal, stable public API that allows azure-functions-durable (and only that package) to replace durable-related converters without touching any private internals of azure-functions.

Only 4 binding names are permitted to be overridden: orchestrationTrigger, entityTrigger, activityTrigger, durableClient.

This does not change:

1. No changes to the cold-start path for non-durable function apps
2. No changes to existing converter registration behavior
3. Converter overrides are the responsibility of azure-functions-durable itself

[andystaples]

FWIW, this works really well for the Durable scenario and removes the ask for PRs like
#353
#309

[Copilot]

Pull request overview

Introduces a new public azure.functions.register_converter() API to let integration packages (notably azure-functions-durable) replace converters for a small allow-list of durable-related bindings, without reaching into private internals.

Changes:

• Add _OVERRIDABLE_BINDINGS allow-list and register_converter() implementation in azure/functions/meta.py.
• Export register_converter from the azure.functions package surface (__init__.py / __all__).
• Add unit tests covering allow-list enforcement and overwrite behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
tests/test_meta.py	Adds tests for the new register_converter API (allow-list + overwrite semantics + export surface).
azure/functions/meta.py	Implements allow-listed converter registration/override API on top of the binding registry.
azure/functions/init.py	Exposes register_converter as part of the public azure.functions API.

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