Add data model payload conversion hooks

[tconley1428]

Adds SDK payload-converter support for values and type hints that expose _temporal_to_data_model / _temporal_from_data_model hooks.

The hooks let a type provide the data-model value that should be handed to the configured payload converter, and reconstruct the original type after the converter decodes that representation. This keeps normal SDK payload-converter behavior in one place, including codec handling and serialization-context propagation, while allowing generated or user-defined wrapper types to model wire shapes that contain nested payload fields.

This matters for signal-with-start and similar workflow-targeted requests because nested user payloads are consumed by a workflow. The hook path lets those payload fields be serialized through the target workflow payload converter context instead of leaking the surrounding Nexus operation context.

This change:

• wraps configured payload converters with an internal data-model adapter when DataConverter creates them
• keeps the adapter internal instead of exporting it as public converter API
• applies the hooks for both encode values and decode type hints before delegating to the configured payload converter
• routes hook-aware outer-envelope conversion through the active payload converter so nested user payload fields preserve SDK serialization behavior
• passes a payload-converter factory into workflow instance details so workflow instances receive already-wrapped converters rather than re-instantiating raw converter classes