rust: add `.rustfmt.toml`

This is the configuration file for the `rustfmt` tool.

`rustfmt` is a tool for formatting Rust code according to style guidelines.
It is very commonly used across Rust projects.

The default configuration options are used.

Reviewed-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>