[OTLP] Add TLS-only support to OTLP HTTP client factory

[sandy2008]

Fixes #6764
Design discussion issue #6764

Changes

Add TLS-only support to the OTLP HTTP client factory while keeping mTLS behavior. Introduces OtlpTlsOptions, updates certificate loading/validation flow for CA-only and mTLS scenarios, and expands tests to cover TLS configuration paths.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.90%. Comparing base (866e218) to head (85b4f5b).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...tation/OpenTelemetryProtocolExporterEventSource.cs	60.00%	2 Missing ⚠️
...ocol/Implementation/OtlpSecureHttpClientFactory.cs	96.49%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6799      +/-   ##
==========================================
+ Coverage   86.88%   86.90%   +0.02%     
==========================================
  Files         262      263       +1     
  Lines       12350    12372      +22     
==========================================
+ Hits        10730    10752      +22     
  Misses       1620     1620              

Flag	Coverage Δ
unittests-Project-Experimental	86.75% <94.59%> (-0.12%)	⬇️
unittests-Project-Stable	86.70% <94.59%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...yProtocol/Implementation/OtlpCertificateManager.cs	76.81% <100.00%> (+0.87%)	⬆️
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	99.27% <100.00%> (ø)
....Exporter.OpenTelemetryProtocol/OtlpMtlsOptions.cs	100.00% <100.00%> (ø)
...y.Exporter.OpenTelemetryProtocol/OtlpTlsOptions.cs	100.00% <100.00%> (ø)
...tation/OpenTelemetryProtocolExporterEventSource.cs	82.17% <60.00%> (-1.16%)	⬇️
...ocol/Implementation/OtlpSecureHttpClientFactory.cs	90.58% <96.49%> (+3.92%)	⬆️

[rajkumar-rangaraj]

@sandy2008 I have provided some important feedback. Please let me know once you have addressed these points.

[sandy2008]

@rajkumar-rangaraj Please kindly review again, tyvm! And happy new year!