[EXPORTER] Allow custom HttpClient in OTLP HTTP

[marcalff]

Fixes #3929

Changes

Please provide a brief description of the changes here.

• In OtlpHttpClient, expose the constructor that accepts an arbitrary ext::http::client::HttpClient implementation as public, instead of private.
• This allows an application to provide its own ext::http::client::HttpClient.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.04%. Comparing base (af66d8d) to head (583d5f0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3930      +/-   ##
==========================================
- Coverage   90.07%   90.04%   -0.02%     
==========================================
  Files         226      226              
  Lines        7226     7226              
==========================================
- Hits         6508     6506       -2     
- Misses        718      720       +2     

see 2 files with indirect coverage changes

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[marcalff]

Disclosure:

My application is using this to support a different authentication to an OTLP HTTP endpoint, contributing the change upstream.

[dbarker]

Looks good to me. Thanks.