Add SDK client library

[sangaline]

This adds a basic sketch of the SDK client library. I mostly mirrored the Python SDK method structure, but would like to diverge from that to make things more ergonomic. Most of the complexity here isn't really in the SDK itself, but rather in the testing framework around it. Making circuit creation requests mockable with Nock required a MITM proxy and some fairly sophisticated request parsing to handle multipart form payloads and the non-determinism of the gzipped tarballs. The circuit creation implementation was also a little tricky in terms of supporting both browsers and node. With these more difficult parts figured out, it shouldn't be too bad to refactor things and clean them up.

Note that the docstrings right now are pure fluff, I barely cleaned up what chatgpt output. I wanted to get something there so that I can test/develop the documentation build and then I'll clean these up and write real docs as I refactor the SDK's API.