feat(ai): add headers option to StreamOptions for custom HTTP headers

- Added headers field to base StreamOptions interface
- Updated all providers to merge options.headers with defaults
- Forward headers and onPayload through streamSimple/completeSimple
- Bedrock not supported (uses AWS SDK auth)

packages/ai/src/stream.ts

@@ -218,6 +218,8 @@ function mapOptionsForApi<TApi extends Api>(
 		signal: options?.signal,
 		apiKey: apiKey || options?.apiKey,
 		sessionId: options?.sessionId,
+		headers: options?.headers,
+		onPayload: options?.onPayload,
 	};
 
 	// Helper to clamp xhigh to high for providers that don't support it