add PLGNFreeCString(str *C.char) to free the memory allocated by Golang using C.CString()

[yushihang]

close #72

[olomix]

Sorry, maybe I'm missing something, but what is the purpose of wrapping the free function in Go's PLGNFreeCString? Why not call it directly?

For now, I don't see any rationale for adding this function.

[yushihang]

Hi @olomix,

Thanks for you response.

Since this PR involves two repositories, I have written a more detailed response in the PR for polygonid-flutter-sdk here: 0xPolygonID/polygonid-flutter-sdk#390 (comment).

In my understanding, PLGNFreeCString() does not only provide a C.free() function for Flutter, but it should be understood as follows:

1. Since the memory is allocated from the OS by Golang, Flutter does not release it directly.
2. Flutter/Dart calls PLGNFreeCString() to inform Golang that the memory block that Golang allocated will not be used anymore by Flutter/Dart.
3. Golang can handle this memory block subsequently. (Golang can then free the memory block by calling free(), or manage the memory block in other ways.)