Add Transfer Types Spec

[VegetarianOrc]

What was changed

• Add generated spec based on Python, .NET, and Java prototype behavior for collaborative review

Why?

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

[dplyukhin]

Suggested change

	## Purpose
	This document defines the portable behavior of Temporal transfer type converters. It specifies observable conversion semantics without prescribing SDK-specific APIs, generic type mechanisms, or converter construction strategies.
	## Motivation
	Sometimes you have a custom datatype that should be serializable as a payload, but you can't modify the configured data converter to support your type. Transfer types let you work around that by picking a "transfer type" that's already serializable, and associating your datatype with a "transfer type converter" that converts values of your custom type to values of the transfer type and back.

[dplyukhin]

Suggested change

	- **Model type:** The user-facing type associated with a transfer type converter.
	- **Transfer type:** The serializer-facing type produced from a model value.
	- **Transfer value:** A value of the transfer type.
	- **Payload conversion:** Conversion between an in-memory value and a Temporal `Payload`.
	- **Payload codec:** A byte-to-byte transformation applied to a payload, such as encryption or compression.
	- **Failure details:** Typed payloads stored in application failures, cancellations, timeout heartbeat details, reset failures, or nested failure causes.
	- **Model type:** The user-facing type associated with a transfer type converter.
	- **Transfer type:** The serializable type produced from a model value.
	- **Transfer value:** A value of the transfer type.

[dplyukhin]

Suggested change

	## Normative behavior
	## Behavior