Client API: Create separate _copy_file method in addition to _move_file

[pjbull]

Personally, I'd find it more intuitive to have separate methods _move_file and _copy_file, where the base implementation of _move_file would be something like:

def _move_file(self, src: BoundedCloudPath, dst: BoundedCloudPath) -> BoundedCloudPath:
    self._copy_file(src, dst)
    if src != dst:
        self._remove(src)
    return dst

Originally posted by @jayqi in #142 (comment)