Mapped VSIMemFile methods

[ChristianBeilschmidt]

• I agree to follow the project's code of conduct.
• I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

---

I mapped the methods VSIFileFromMemBuffer, VSIUnlink, VSIGetMemFileBuffer.
Since I haven't found a good place to put them, I created a new vsi module in the root.

There are two methods for creation and retrieval that work with owned data, such that either we give the ownership from our Rust object to GDAL's CPP context or vice versa.

Moreover, there are two methods that work with references and they do not hand over ownership. This is more complicated since we don't want to introduce memory leaks. I created a retrieval method that only gets a function so that we do not hand out data but only a computation result from the function. For creation, I introduced a MemFileRef struct that handles the lifetime to the referenced data.

It would be nice if someone would check that my solution makes sense in that case.