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.

[Quinn-With-Two-Ns]

Why build the options then immediately call ScheduleClientOptions.newBuilder(options)?

[VegetarianOrc]

I took a pass at cleaning this up a little by passing the data converter into the invoker rather than pulling it from the options. Not 100% sure that's a reasonable approach so happy to consider a better alternative here if there is one.

[Quinn-With-Two-Ns]

We took a very different approach to wrapping the users DataConverter here vs externalStorage especially for client calls, was there a reason we needed to diverge I would kind of expect both to have the same requirements ?

[VegetarianOrc]

We took a very different approach to wrapping the users DataConverter here vs externalStorage especially for client calls, was there a reason we needed to diverge I would kind of expect both to have the same requirements ?

Thanks for calling this out. I've taken a pass to consolidate the client side converter creation into ClientDataConverterFactory (renamed from WorkflowClientDataConverterFactory). I think it's closer to what you had in mind, but let me know if I'm off.

I've left the worker side separate for now since the transfer type conversion happens in a workflow thread but the external storage must remain separate on that side. There may still be cleanup possible that I can explore if you think it could use some attention.