chore: add cross_sync annotations

[daniel-sanche]

PR 2/x for adding the sync surface for the new data client

This PR restructures the existing async library to better support the sync side. Specifically:

• all async-specific calls and types are replaced with corresponding calls to the new CrossSync class, which simply contains a set of aliases pointing back at the corresponding async functionality
• all async keywords are annotated with CrossSync.rm_aio, which tells the ast transformers where to strip out asyncio keywords

Currently, we need to run the conversion process manually (using python .cross_sync/generate.py, and the sync files are not included in the codebase. The next PR will add the sync artifacts, and add the generation pipeline into the CI system

[daniel-sanche]

Is there a test to validate that the sync surface is re-generated when there's change to async code?

Yes, but that's in the next PR, since we don't have the generated files here yet