http: add writeInformation to send arbitrary 1xx status codes

[pimterry]

This provides a generic HTTP/1 API for 1xx responses, equivalent to our existing HTTP/2 additionalHeaders API. This supports any 1xx status code, except 101, which I've intentionally excluded - since it changes the protocol entirely, there's no possible way to unilaterally send that without breaking things later in our current API, so you just have to use 'upgrade' in that case.

This fixes various past requests like:

• Make http.OutgoingMessage._writeRaw public #22624
• Expose and write headers on 1xx intermediate status codes #27921

And acts as a nice workaround to solve:

• Feature request: passing headers to response.writeContinue() #7588

(All 3 closed as stale, not fixed)

I'm also hoping it'll discourage usage of the private res._writeRaw method as a workaround for the existing limitations. We even did this in our own tests, see test/parallel/test-http-information-headers.js here.

This is useful to support custom 1xx values (perfectly valid AFAICT, if unusual), proxies (who want to pass through data in a standard way, without having to separately handle each case individually as today) and to give us trivial general support for future standardized 1xx values, without needing a whole separate PR to add a new per-status API every time.

The existing methods are refactored to use the generic method under the hood, and HTTP/2 compat uses additionalHeaders.

There's one annoying inconsistency here: the H1 methods throw if we've already sent a final status code, while the H2 methods just no-op. I've kept that as-is and matched this in the new method to avoid a breaking change, but it would probably be best to align on throwing in both cases in a separate major bump.

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/http2
• @nodejs/net

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.02%. Comparing base (b5da751) to head (af03c6b).

Files with missing lines	Patch %	Lines
lib/_http_server.js	85.45%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63155      +/-   ##
==========================================
- Coverage   90.03%   90.02%   -0.02%     
==========================================
  Files         713      713              
  Lines      224510   224565      +55     
  Branches    42438    42462      +24     
==========================================
+ Hits       202148   202161      +13     
- Misses      14163    14222      +59     
+ Partials     8199     8182      -17     

Files with missing lines	Coverage Δ
lib/internal/http2/compat.js	97.10% <100.00%> (+0.13%)	⬆️
lib/_http_server.js	96.61% <85.45%> (-0.33%)	⬇️

... and 31 files with indirect coverage changes

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[metcoder95]

Don't have approval power, but lgtm; nice addition

[pimterry]

Rebased to hopefully fix the build now that the QUIC coverage issue is sorted.