Add transfer type converters

[VegetarianOrc]

What changed?

• Adds experimental @TransferTypeConvertible and TransferTypeConverter APIs for converting top-level model values to a transfer representation before data conversion.
• Applies transfer conversion across workflow, activity, Nexus, schedule, test activity environment, and workflow stream paths.
• Adds workflow-stream item conversion support, documentation, and coverage for converter behavior, codecs, and SDK integration points.

Why?

• Lets applications define a model-owned transfer representation while retaining the configured data converter for payload serialization, codecs, and wire encoding.

Breaking changes?

• No.

Server PR

• No.