fix(env-http-proxy-agent): assume http for schema-less proxy env vars

[nthbotast]

Summary

• assume an http:// scheme when HTTP_PROXY / HTTPS_PROXY values are provided without a URL scheme
• keep existing behavior unchanged for already well-formed proxy URLs
• add coverage for schema-less proxy environment variables in EnvHttpProxyAgent tests

Testing

• node --test test/env-http-proxy-agent.js

Closes #4736

[metcoder95]

What's the rationale of normalizing https to http?
unless tunneling, that can cause connect rejection from proxy.

[metcoder95]

Let's do that.

I can see a bit of ambiguity if I try to set a schema less origin proxy.example that I know only accepts TLS traffic and starts attempting plain text connection.
On top, it seems is what also curl does.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.14%. Comparing base (f734c87) to head (d023525).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4860   +/-   ##
=======================================
  Coverage   93.13%   93.14%           
=======================================
  Files         109      109           
  Lines       34243    34258   +15     
=======================================
+ Hits        31893    31909   +16     
+ Misses       2350     2349    -1     

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