Refine must2 and apply NewAesGcm() to all usage

common/common.go

@@ -23,7 +23,9 @@ func Must(err error) {
 }
 
 // Must2 panics if the second parameter is not nil, otherwise returns the first parameter.
-func Must2(v interface{}, err error) interface{} {
+// This is useful when function returned "sth, err" and avoid many "if err != nil"
+// Internal usage only, if user input can cause err, it must be handled
+func Must2[T any](v T, err error) T {
 	Must(err)
 	return v
 }