Add asinfo package for AS number information lookups

- Created pkg/asinfo with embedded AS data from ipverse/asn-info
- Provides fast lookups by ASN with GetDescription() and GetHandle()
- Includes Search() functionality for finding AS by name/handle
- Added asinfo-gen tool to fetch and convert CSV data to JSON
- Added 'make asupdate' target to refresh AS data
- Embedded JSON data contains 130k+ AS entries
- Added comprehensive tests and examples

Makefile

@@ -1,6 +1,6 @@
 export DEBUG = routewatch
 
-.PHONY: test fmt lint build clean run
+.PHONY: test fmt lint build clean run asupdate
 
 all: test
 
@@ -22,3 +22,8 @@ clean:
 
 run: build
 	./bin/routewatch
+
+asupdate:
+	@echo "Updating AS info data..."
+	@go run cmd/asinfo-gen/main.go > pkg/asinfo/asdata.json.tmp && \
+		mv pkg/asinfo/asdata.json.tmp pkg/asinfo/asdata.json