RUM-15682: Support legacy okhttp network telemetry

[satween]

What does this PR do?

Add LEGACY_OKHTTP to the InternalTelemetryEvent.ApiUsage.NetworkInstrumentation.LibraryType enum and to the TelemetryUsageEvent schema, and have DatadogInterceptor.onSdkInstanceReady report the legacy okhttp instrumentation type. Previously only OKHTTP (plugin-based) and CRONET were tracked.

Motivation

Backend telemetry needs to distinguish apps still using the legacy DatadogInterceptor path from those that migrated to the new plugin-based okhttp instrumentation, so we can measure migration progress and make informed deprecation decisions.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.99%. Comparing base (6bc1364) to head (de66c0e).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...droid/internal/telemetry/InternalTelemetryEvent.kt	0.00%	2 Missing ⚠️
...n/com/datadog/android/okhttp/DatadogInterceptor.kt	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3399      +/-   ##
===========================================
- Coverage    72.06%   71.99%   -0.07%     
===========================================
  Files          957      957              
  Lines        35333    35266      -67     
  Branches      5863     5849      -14     
===========================================
- Hits         25461    25388      -73     
- Misses        8261     8276      +15     
+ Partials      1611     1602       -9     

Files with missing lines	Coverage Δ
...ndroid/telemetry/internal/TelemetryEventHandler.kt	87.78% <ø> (+0.39%)	⬆️
...n/com/datadog/android/okhttp/DatadogInterceptor.kt	81.97% <75.00%> (+0.30%)	⬆️
...droid/internal/telemetry/InternalTelemetryEvent.kt	20.45% <0.00%> (-0.48%)	⬇️

... and 35 files with indirect coverage changes

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