Add BearerTokenFunc to support dynamic bearer token authentication
for OTLP exporters. Tokens are injected per-request via gRPC
PerRPCCredentials and HTTP custom RoundTripper.
- Add BearerTokenFunc type and Config field in tracerconfig
- Implement bearerCredentials (gRPC) and bearerRoundTripper (HTTP)
- Wire bearer auth into all exporter creation functions
- Add getHTTPClient helper for DRY HTTP client configuration
- Upgrade OpenTelemetry SDK to v1.39.0 for WithHTTPClient support
