feat(appkit): add plugin trace header hooks and auto Databricks auth for OTLP trace exporter

[hubertzub-db]

Summary

• Plugin trace header contribution: Plugins can now define a static appendTraceHeaders(config) method on their constructor. These headers are collected during _createApp and forwarded exclusively to the OTLP trace exporter, enabling plugins (e.g. MLflow) to inject experiment IDs or other metadata into trace exports.
• Automatic Databricks authentication for traces: When DATABRICKS_HOST is set and no explicit authorization header is provided, TelemetryManager auto-resolves credentials via WorkspaceClient and attaches them to the trace exporter. This enables direct trace export to Databricks-hosted OTLP endpoints without manual token configuration.
• Lazy instrumentation factories: HTTP and Express instrumentations are now created via factory functions instead of being eagerly instantiated at import time, preventing premature module patching side effects.
• Server telemetry opt-out: Added enableDefaultTelemetry option to ServerConfig, allowing consumers to disable automatic HTTP/Express span creation when not needed.