ci, maint: build/publish to PyPI, build linux/mac and amd64/arm64 wheels

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Summary

• We now have automation to publish packaged source distributions and wheels to PyPI.
• We provide four platform dependent wheels for dask-gateway-server for linux and mac that comes in amd64 and arm64 variants.
• Various refactoring is done and documentation is improved to reduce complexity and improve long term maintenance.
• This workflow was developed against PyPI's test server, see https://test.pypi.org/project/dask-gateway-server/0.9.0/#files for the result.

Noteworthy details

• dask-gateway-server/build_tools are removed as most relevant logic moved to dask-gateway-server/setup.py and the remaining logic to set GOOS and GOARCH Golang cross compilation environment variables made sense to do in the GitHub Workflow without having a dedicated script for.
• dask-gateway-server's --no-build-proxy flag in setup.py is replaced with an environment variable DASK_GATEWAY_SERVER__NO_PROXY. Retaining the flag forced many not-robust workarounds. The environment variable also helps us ensure we don't package a binary in wheels we build.

Closes #537

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I'll go for a merge and trial a pre-release to verify functionality of the CI system and if it succeeds, follow up with tests against the release end-to-end in a real world installation.