feat: allow reusing an existing OkHttpClient via DefaultHttpClient.Bu…

[tanya732]

Summary

Adds DefaultHttpClient.Builder.withClient(OkHttpClient), letting integrators reuse an existing OkHttpClient as the base for the Authentication API HTTP client.

The behavior boundary is deliberate and documented:

• The supplied client owns all transport configuration, timeouts, dispatcher, connection pool, cache, proxy, and its own interceptors are reused as-is via baseClient.newBuilder(). The builder's transport setters (withReadTimeout, withConnectTimeout, withMaxRequests, withMaxRequestsPerHost, withProxy) are ignored when a base client is supplied, so the caller's client is never silently overridden. Each of those setters now carries a @see #withClient note.
• The SDK always layers its behavior on top telemetry, rate-limit handling, and logging interceptors are added to the caller's client and cannot be bypassed; they remain configurable via withTelemetry / telemetryEnabled / withMaxRetries / withLogging.
• The default path (no base client) is unchanged, existing users get identical behavior, no breaking change.

The Management API already accepts a raw OkHttpClient, so no generated code is touched.

References

• Issue Make DefaultHttpClient extensible #754

Testing

• New DefaultHttpClientTest (added to .fernignore) covering: base-client transport reused / builder transport ignored, default path still applies builder transport, SDK interceptors layered without mutating the original, null base client throws.
• AuthAPITest integration test asserting a base-client interceptor header and the Auth0 telemetry header both appear.
• ./gradlew spotlessApply check passes.

Checklist

• Backward compatible
• Tests added
• Docs updated (EXAMPLES.md)

[rmad17]

LGTM!