rust: str: implement `Index` for `BStr`

The `Index` implementation on `BStr` was lost when we switched `BStr` from
a type alias of `[u8]` to a newtype. Add back `Index` by implementing
`Index` for `BStr` when `Index` would be implemented for `[u8]`.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Daniel Gomez <da.gomez@samsung.com>
Link: https://lore.kernel.org/r/20250227-module-params-v3-v8-2-ceeee85d9347@kernel.org


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