Add GeoIP and GeoSite support for expr (#38)

* feat: copy something from hysteria/extras/outbounds/acl

* feat: add geoip and geosite support for expr

* refactor: geo matcher

* fix: typo

* refactor: geo matcher

* feat: expose config options to specify local geoip/geosite db files

* refactor: engine.Config should not contains geo

* feat: make geosite and geoip lazy downloaded

* chore: minor code improvement

* docs: add geoip/geosite usage

---------

Co-authored-by: Toby <tobyxdd@gmail.com>

ruleset/builtins/geo/v2geo/load.go

@@ -0,0 +1,44 @@
+package v2geo
+
+import (
+	"os"
+	"strings"
+
+	"google.golang.org/protobuf/proto"
+)
+
+// LoadGeoIP loads a GeoIP data file and converts it to a map.
+// The keys of the map (country codes) are all normalized to lowercase.
+func LoadGeoIP(filename string) (map[string]*GeoIP, error) {
+	bs, err := os.ReadFile(filename)
+	if err != nil {
+		return nil, err
+	}
+	var list GeoIPList
+	if err := proto.Unmarshal(bs, &list); err != nil {
+		return nil, err
+	}
+	m := make(map[string]*GeoIP)
+	for _, entry := range list.Entry {
+		m[strings.ToLower(entry.CountryCode)] = entry
+	}
+	return m, nil
+}
+
+// LoadGeoSite loads a GeoSite data file and converts it to a map.
+// The keys of the map (site keys) are all normalized to lowercase.
+func LoadGeoSite(filename string) (map[string]*GeoSite, error) {
+	bs, err := os.ReadFile(filename)
+	if err != nil {
+		return nil, err
+	}
+	var list GeoSiteList
+	if err := proto.Unmarshal(bs, &list); err != nil {
+		return nil, err
+	}
+	m := make(map[string]*GeoSite)
+	for _, entry := range list.Entry {
+		m[strings.ToLower(entry.CountryCode)] = entry
+	}
+	return m, nil
+}