http: send SNI when connecting to https proxies

[lazerg]

When the proxy URL uses https://, the http and https agents connect to the proxy with tls.connect() using only the host and port from ProxyConfig, so no SNI is sent. Proxies that choose their certificate by SNI then answer with their default one, which can fail verification. This sets servername to the proxy host when it isn't an IP address, the same way undici does for fetch().

Fixes: #66718

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.49%. Comparing base (87db28b) to head (5d37cfa).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66720      +/-   ##
==========================================
- Coverage   92.82%   90.49%   -2.34%     
==========================================
  Files         422      791     +369     
  Lines      194297   277208   +82911     
  Branches    30063    53305   +23242     
==========================================
+ Hits       180359   250847   +70488     
- Misses      13613    16759    +3146     
- Partials      325     9602    +9277     

Files with missing lines	Coverage Δ
lib/internal/http.js	92.59% <100.00%> (+0.10%)	⬆️

... and 499 files with indirect coverage changes

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