[OTLP] Decoupling OTEL_EXPORTER_OTLP_CERTIFICATE from mTLS

[sandy2008]

Fixes #6764
Design discussion issue #6764

Changes

Decouples OTEL_EXPORTER_OTLP_CERTIFICATE from mTLS-specific classes to support standalone TLS scenarios (e.g., trusting self-signed server certificates without client authentication).

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 90.00000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.80%. Comparing base (1080291) to head (9c32859).
⚠️ Report is 13 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ocol/Implementation/OtlpSecureHttpClientFactory.cs	90.90%	8 Missing ⚠️
...tation/OpenTelemetryProtocolExporterEventSource.cs	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6777      +/-   ##
==========================================
+ Coverage   86.71%   86.80%   +0.08%     
==========================================
  Files         262      265       +3     
  Lines       12319    12699     +380     
==========================================
+ Hits        10683    11023     +340     
- Misses       1636     1676      +40     

Flag	Coverage Δ
unittests-Project-Experimental	86.66% <90.00%> (-0.05%)	⬇️
unittests-Project-Stable	86.76% <90.00%> (+0.17%)	⬆️

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	75.93% <ø> (ø)
...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	90.90% <90.90%> (ø)

... and 12 files with indirect coverage changes

[rajkumar-rangaraj]

Would you be open to splitting this into two PRs?

1. A pure refactor (renames + reorganization) with no behavior changes
2. The new TLS-only feature on top of that

This would make review easier and give us a chance to discuss the API surface (e.g., the MtlsOptions property name for non-mTLS scenarios) separately from the mechanical changes.

[sandy2008]

Would you be open to splitting this into two PRs?

1. A pure refactor (renames + reorganization) with no behavior changes
2. The new TLS-only feature on top of that

This would make review easier and give us a chance to discuss the API surface (e.g., the MtlsOptions property name for non-mTLS scenarios) separately from the mechanical changes.

@rajkumar-rangaraj Sure :) I will do it.