Adds experimental SDK payload-converter support for values and type hints decorated with @transfer_type_convertible(...) using a TransferTypeConverter class.

The converter maps a user value to the transfer type handed to the configured payload converter, and maps the decoded transfer type back to the user value. This keeps normal SDK payload-converter behavior in one place, including codec handling and serialization-context propagation.

The general transfer type hook is intentionally pure and does not receive a payload converter. System Nexus conversion obtains its configured user payload converter through private SDK context when generated system models need to process nested user payloads.

This change:

• adds the experimental public TransferTypeConverter ABC and transfer_type_convertible(...) decorator
• wraps configured payload converters with an internal transfer type adapter when DataConverter creates them
• applies registered transfer type converters for both encoded values and decoded type hints before delegating to the configured payload converter
• supports an optional TransferTypeConverter.transfer_type decode hint; when absent, the wrapped converter decodes without an inner type hint
• keeps the payload converter adapter and System Nexus support helpers private
• passes a payload-converter factory into workflow instance details so workflow instances receive already-wrapped converters rather than re-instantiating raw converter classes

Breaking changes:

• Custom workflow runners that construct WorkflowInstanceDetails must now pass payload_converter_factory instead of payload_converter_class.
• System Nexus payload converter helpers added for generated bindings are private implementation details. The remaining public temporalio.nexus.system APIs are experimental and subject to change.

Validation:

• ./.venv/bin/pytest tests/test_converter.py -q
• poe lint