Add reference documentation for c8y_auth_proxy

[jarhodes314]

Proposed changes

This PR adds some public facing documentation for c8y_auth_proxy, so customers know it exists and can make use of it if they wish. It also improves the proxy's handling of large bodies, proactively checking the JWT is valid before making the real request.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

#2174

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

Merging #2330 (cfba699) into main (4fc84df) will increase coverage by 0.4%.
Report is 22 commits behind head on main.
The diff coverage is 96.9%.

Additional details and impacted files

Files	Coverage Δ
crates/extensions/c8y_auth_proxy/src/server.rs	91.1% <96.9%> (+6.7%)	⬆️

... and 31 files with indirect coverage changes

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
314	0	3	314	100	59m25.282s