feat(rust/wrappers): init `BcachefsHandle`

feat(rust/wrappers): init `BcachefsHandle`

We propose a simple low-level wrapper which can perform various subvolume-related operations
as an example for the API surface.

This will be used in an upcoming commit to migrate the subvolume CLI fully to Rust.

The API design is the following:

- `BcachefsHandle` is meant as a low level handle to carry around whenever you need a filesystem handle
  to send ioctl to.
- it possess type-safe operations

Type safe operations are handled by having type safe wrappers for ioctl commands
*and* their payloads.

We assume that all ioctl payloads only use *one* argument, this can easily be changed if needed.

When the handle goes out of scope, we automatically close it à la C++ RAII.

Signed-off-by: Ryan Lahfa <bcachefs@lahfa.xyz>