[Feature Request]: Serialize Python user types with Beam schemas

### What would you like to happen?

#16558 proposed adding an `sdk_options` attribute to the Beam schema proto allowing SDKs to encode serialized user types. We should revive that PR, and use it to serialize the (Python) user type that will be used to represent the schema (or from which the schema was inferred).

This will enable users to utilize methods on their types, and will make it simpler for us to add support for inferring from other user types (dataclasses, TypedDicts, etc..).

### Issue Priority

Priority: 2

### Issue Component

Component: sdk-py-core