mpl proxy for matplotlib process

[maxtheman]

📝 Summary

This represents the matplotlib fix to create a proxy middleware which connects the main starlette process to the mpl starlette process.

Using this mapplotlib can be used in production on Docker for any kind of interactive stuff.

See my demo app here, which is finally working: https://causal-quartets.fly.dev/

I left the environmental variables in the codebase so that you could customize the port or hosts for some reason in the future, but I removed references to them from the documentation since I think that would be more of a developer-facing thing anyway.

🔍 Description of Changes

1. Creates AsyncHTTPClient class to manage HTTP responses between the two starlette processes without any dependencies (i.e., no httpx)
2. Fixes The WebSocket handling from the original implementation of the proxy class. There were two things wrong: The websocket proxy was dropping the figure url params required for matplotlib to find the figure. There was also a bare Exception that was silently catching the assertion failure in _mpl, which made diagnosis difficult.
3. adds 9 tests which successfully simulate and range the gamut across all of the issues we were seeing and make sure that all of the HTTP and WebSocket methods we're talking about work correctly as well as testing it directly with the mpl middleware.

📋 Checklist

• [x] I have read the contributor guidelines.
• [x] For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• [x] I have added tests for the changes made.
• [x] I have run the code and verified that it works as expected.

📜 Reviewers

@mscolnick - finally ready for you

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[maxtheman]

I have read the CLA Document and I hereby sign the CLA