feat: implement structured logging system throughout the codebase

- Add logger utility adapted from payload-mailing pattern
- Use PAYLOAD_BILLING_LOG_LEVEL environment variable for configuration
- Replace console.* calls with contextual loggers across providers
- Update webhook utilities to support proper logging
- Export logging utilities for external use
- Maintain fallback console logging for compatibility

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/index.ts

@@ -5,6 +5,9 @@ export type { BillingPluginConfig, CustomerInfoExtractor, AdvancedTestProviderCo
 export type { Invoice, Payment, Refund } from './plugin/types/index.js'
 export type { PaymentProvider, ProviderData } from './providers/types.js'
 
+// Export logging utilities
+export { getPluginLogger, createContextLogger } from './utils/logger.js'
+
 // Export all providers
 export { testProvider } from './providers/test.js'
 export type {