common/helpers: convert SubnetMap to github.com/gaissmai/bart

I did not benchmark it myself, but it was benchmarked here:
 https://github.com/osrg/gobgp/issues/1414#issuecomment-3067255941

Of course, no guarantee that this benchmark matches our use cases.
Moreover, SubnetMap have been optimized to avoid parsing keys all
the time.

Also, the interface is a bit nicer and it uses netip.Prefix directly.

The next step is to convert outlet/routing/provider/bmp.

common/helpers/tests_diff.go

@@ -30,6 +30,7 @@ func defaultPrettyFormatters() map[reflect.Type]any {
 	result := map[reflect.Type]any{
 		reflect.TypeOf(net.IP{}):            fmt.Sprint,
 		reflect.TypeOf(netip.Addr{}):        fmt.Sprint,
+		reflect.TypeOf(netip.Prefix{}):      fmt.Sprint,
 		reflect.TypeOf(time.Time{}):         fmt.Sprint,
 		reflect.TypeOf(SubnetMap[string]{}): fmt.Sprint,
 		reflect.TypeOf(SubnetMap[uint]{}):   fmt.Sprint,