Reactivate withTrustedRoots by default in HTTP binding

[JKRhb]

While #194 introduced the ability to set custom trusted certificates for the security contexts that are being created from the HTTP binding, it accidentally let the HTTP clients not use the default trusted root certificates of the underlying platform anymore. This PR fixes the issue by introducing a new config parameter, partly by using it in an appropriate place.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 63.45%. Comparing base (a3156ac) to head (e7dc1fe).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/binding_http/http_client.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
- Coverage   63.47%   63.45%   -0.03%     
==========================================
  Files          78       78              
  Lines        2300     2301       +1     
==========================================
  Hits         1460     1460              
- Misses        840      841       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.