rust: introduce `.clippy.toml`

Some Clippy lints can be configured/tweaked. We will use these knobs to
our advantage in later commits.

This is done via a configuration file, `.clippy.toml` [1]. The file is
currently unstable. This may be a problem in the future, but we can adapt
as needed. In addition, we proposed adding Clippy to the Rust CI's RFL
job [2], so we should be able to catch issues pre-merge.

Thus introduce the file.

Link: https://doc.rust-lang.org/clippy/configuration.html [1]
Link: https://github.com/rust-lang/rust/pull/128928

 [2]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-12-ojeda@kernel.org


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