hysteria: also accept "hysteria2" protocol string

UI stores v1 and v2 both as "hysteria" with settings.version, but
inbounds that came in from imports / manual SQL can carry the
literal "hysteria2" string and get silently dropped everywhere we
switch on protocol.

Add Hysteria2 constant + IsHysteria helper, use it in the places
that gate on protocol (sub SQL, getLink, genHysteriaLink, clash
buildProxy, json gen, inbound.go validation, xray AddUser).

Existing "hysteria" inbounds are untouched.

closes #4081

sub/subJsonService.go

@@ -209,7 +209,7 @@ func (s *SubJsonService) getConfig(inbound *model.Inbound, client model.Client,
 			newOutbounds = append(newOutbounds, s.genVless(inbound, streamSettings, client))
 		case "trojan", "shadowsocks":
 			newOutbounds = append(newOutbounds, s.genServer(inbound, streamSettings, client))
-		case "hysteria":
+		case "hysteria", "hysteria2":
 			newOutbounds = append(newOutbounds, s.genHy(inbound, newStream, client))
 		}