Support saving to buffer

The current bindings only allow to save to a file.
For our use case, we need to parse and remove exif tags in memory, for all kinds of images.
=> We would need to be able to save the resulting images (after removing tags), to a buffer (ideally using [`Write`](https://doc.rust-lang.org/stable/std/io/trait.Write.html)?).

It looks like this might be possible using https://gitlab.gnome.org/GNOME/gexiv2/-/blob/e4d65b31cd77f28ef248117e161de9d8cc31d712/gexiv2/gexiv2-metadata.cpp#L715 or https://gitlab.gnome.org/GNOME/gexiv2/-/blob/e4d65b31cd77f28ef248117e161de9d8cc31d712/gexiv2/gexiv2-metadata.cpp#L592, but is not currently available through the rust libraries.