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

xray/api.go

@@ -231,7 +231,7 @@ func (x *XrayAPI) AddUser(Protocol string, inboundTag string, user map[string]an
 				Email: userEmail,
 			})
 		}
-	case "hysteria":
+	case "hysteria", "hysteria2":
 		auth, err := getRequiredUserString(user, "auth")
 		if err != nil {
 			return err