rust: str: introduce `NullTerminatedFormatter`

rust: str: introduce `NullTerminatedFormatter`

Add `NullTerminatedFormatter`, a formatter that writes a null terminated
string to an array or slice buffer. Because this type needs to manage the
trailing null marker, the existing formatters cannot be used to implement
this type.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250902-rnull-up-v6-16-v7-4-b5212cc89b98@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>