http2: add diagnostics channels for client stream request body

[RaisinTen]

These would allow inspection of HTTP/2 client stream request bodies.

[nodejs-github-bot]

Review requested:

• @nodejs/http2
• @nodejs/net

[codecov]

Codecov Report

❌ Patch coverage is 86.11111% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.56%. Comparing base (ba7cdf4) to head (939ecff).
⚠️ Report is 30 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/http2/core.js	86.11%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60480      +/-   ##
==========================================
- Coverage   88.56%   88.56%   -0.01%     
==========================================
  Files         704      704              
  Lines      207774   207864      +90     
  Branches    40027    40047      +20     
==========================================
+ Hits       184020   184090      +70     
- Misses      15800    15815      +15     
- Partials     7954     7959       +5     

Files with missing lines	Coverage Δ
lib/internal/http2/core.js	95.12% <86.11%> (-0.09%)	⬇️

... and 42 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/69946/

[IlyasShabi]

Isn’t it better to preserve the original chunk type (string, Buffer, TypedArray) instead of always converting it to a Buffer? This avoids extra allocation and encoding costs, and defers type conversion to the subscriber.