Propogate field_descriptions to RowTypeConstraint

[Polber]

#29561 introduced the ability to persist Field descriptions defined in Cross-language transform configs as shown here:

beam/sdks/python/apache_beam/typehints/schemas.py

Lines 546 to 570 in 80c7450

	descriptions = {}
	for field in schema.fields:
	try:
	field_py_type = self.typing_from_runner_api(field.type)
	if isinstance(field_py_type, row_type.RowTypeConstraint):
	field_py_type = field_py_type.user_type
	except ValueError as e:
	raise ValueError(
	"Failed to decode schema due to an issue with Field proto:\n\n"
	f"{text_format.MessageToString(field)}") from e
	descriptions[field.name] = field.description
	subfields.append((field.name, field_py_type))
	user_type = NamedTuple(type_name, subfields)
	# Define a reduce function, otherwise these types can't be pickled
	# (See BEAM-9574)
	setattr(
	user_type,
	'__reduce__',
	_named_tuple_reduce_method(schema.SerializeToString()))
	setattr(user_type, "_field_descriptions", descriptions)
	setattr(user_type, row_type._BEAM_SCHEMA_ID, schema.id)

However, this information is dropped when converting the NamedTuple (parsed from the proto) into a RowTypeConstraint type.

This PR propagates this optional data so that when converting from RowTypeConstraint to schema_pb2.Schema (runner_api), the field descriptions can be included in the proto.

---

This is also helpful for Beam YAML auto documentation which attempts to use the description field from the schema_pb2.Schema, but is not populated by default. After this change, the auto documentation can pull in decsriptions for cross-language (Java) transforms annotated with @SchemaFieldDescription

---

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[Polber]

R: @robertwb

This PR fixes the issue with YAML auto docs not pulling in Java transform parameter descriptions.

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