Move `ClientContext` under `RequestOptions`

[parthea]

PR #1499 included changes to several generated files.

• google/cloud/spanner_v1/types/spanner.py
• google/cloud/spanner_v1/types/__init__.py

#1499 hasn't been released yet, so we could consider the following fixes:

For google/cloud/spanner_v1/types/spanner.py, it would be better to move ClientContext to a handwritten file. If we can also avoid making handwritten changes to google/cloud/spanner_v1/types/__init__.py , then it will be easier to maintain this library using the client library automated tooling.

[parthea]

After closer look, ClientContext exists in generated code, but the location is different. It's under the RequestOptions message.

python-spanner/google/cloud/spanner_v1/types/spanner.py

Lines 404 to 421 in c37532c

	class ClientContext(proto.Message):
	r"""Container for various pieces of client-owned context
	attached to a request.
	Attributes:
	secure_context (MutableMapping[str, google.protobuf.struct_pb2.Value]):
	Optional. Map of parameter name to value for this request.
	These values will be returned by any SECURE_CONTEXT() calls
	invoked by this request (e.g., by queries against
	Parameterized Secure Views).
	"""
	secure_context: MutableMapping[str, struct_pb2.Value] = proto.MapField(
	proto.STRING,
	proto.MESSAGE,
	number=1,
	message=struct_pb2.Value,
	)

https://github.com/googleapis/googleapis/blob/94bc991b3f1df58bc841801631715894a7ac5e28/google/spanner/v1/spanner.proto#L489-L527

I'll create a PR to propose moving it under RequestOptions and revert the changes to google/cloud/spanner_v1/types/__init__.py

[parthea]

This will be fixed in #1529

[aseering]

Regarding ClientContext: Because the corresponding feature hasn't launched yet (neither Client support nor in the backend), I think it's fine to make changes like moving it to under RequestContext. I don't have any particular objection to making this specific change, as long as doing so aligns us with how other classes are handled and is representative of a coding pattern that we'd be planning to stick with going forward.

The choice to "hand-write" this code wasn't deliberate: I'm pretty sure that it actually was generated, it was just generated using a non-standard process for transient reasons that no longer really matter.