http: create header field names as internalized strings

[araujogui]

Internalize header field names so repeated occurrences resolve to the same string in the isolate's string table, speeding up matchKnownFields() comparisons and req.headers builds.

Benchmark

http/incoming_headers.js duration=5 w=0 headers=20 connections=50 benchmarker='wrk'         **      3.20 %       ±2.00% ±2.66% ±3.47%
http/incoming_headers.js duration=5 w=6 headers=20 connections=50 benchmarker='wrk'        ***      3.75 %       ±1.99% ±2.65% ±3.45%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 2 comparisons, you can thus expect the following amount of false-positive results:
  0.10 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.02 false positives, when considering a   1% risk acceptance (**, ***),
  0.00 false positives, when considering a 0.1% risk acceptance (***)

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[Copilot]

Pull request overview

This PR optimizes HTTP header processing by internalizing header field name strings so repeated header names can compare faster (e.g., in JS matchKnownFields()) and reduce duplicate string allocations when building req.headers.

Changes:

• Add StringPtr::ToInternalizedString() to create V8 internalized strings from header field names.
• Use internalized strings for header field names when constructing the headers array passed to JS.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.