Make wasm GIT_CORS_PROXY env var much more configurable

[ianthomas23]

Make wasm GIT_CORS_PROXY env var much more configurable with a variety of substitutions for the url, host, api_key, etc. For example, for the public CORS proxy at https://corsproxy.io/ you could use:

export GIT_CORS_PROXY='https://corsproxy.io/?key={api_key}&url={url:encode}'export GIT_CORS_PROXY_API_KEY=whatever

Needs rigorous downstream testing after merging. Documentation explaining the options will follow in a separate PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.52%. Comparing base (68ae681) to head (2aa0e5a).

Additional details and impacted files

@@ Coverage Diff @@## main #155 +/- ##
==========================================
- Coverage 88.55% 88.52% -0.04% 
==========================================
Files 62 62 Lines 2884 2884 Branches 358 358 ==========================================
- Hits 2554 2553 -1 - Misses 330 331 +1 

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