rust: str: implement `AsRef<BStr>` for `[u8]` and `BStr`

Implement `AsRef<BStr>` for `[u8]` and `BStr` so these can be used
interchangeably for operations on `BStr`.

Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Tested-by: Daniel Gomez <da.gomez@samsung.com>
Link: https://lore.kernel.org/r/20250227-module-params-v3-v8-3-ceeee85d9347@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>