feat: expand PointerTo[T]() function to support proto enums

loosen the generic type constraint on `PointerTo[T]()` to replace `int32` with `~int32` so that we include generated enum types
add additional clarifying comments

helpers_generic.go

@@ -6,13 +6,22 @@ package csproto
 // NativeTypes defines a generic constraint for the native Go types that need to be converted to
 // pointers when storing values in generated Protobuf message structs.
 //
+// The int32 constraint is "expanded" to ~int32 to support generated Protobuf enum types, which always
+// have an underlying type of int32.  While this technically means we will accept types that are not,
+// in fact, Protobuf enums, the utility of making things work for enums outweighs the potential downside
+// of developers intentionally using [PointerTo] to take a pointer to a custom type that happens to
+// be based on int32.
+//
 // Unsized integers (type int) also need to be converted to a pointer, but Protobuf doesn't support
-// unsized integers.  Use csproto.Int(v int) *int32 instead.
+// unsized integers.  Use the [csproto.Int] function instead.
 type NativeTypes interface {
-	bool | int32 | int64 | uint32 | uint64 | float32 | float64 | string
+	bool | ~int32 | int64 | uint32 | uint64 | float32 | float64 | string
 }
 
-// PointerTo makes a copy of v and returns a pointer to that copy
+// PointerTo makes a copy of v and returns a pointer to that copy.
+//
+// The [NativeTypes] type constraint restricts this function to only types that are valid for Protobuf
+// scalar field values (boolean, integer, float, string, and Protobuf enum).
 func PointerTo[T NativeTypes](v T) *T {
 	return &v
 }