Add a rustfmt.toml to the repository (#10809)

According to
https://doc.rust-lang.org/nightly/edition-guide/rust-2024/rustfmt-style-edition.html#migration
and https://github.com/rust-lang/rustfmt?tab=readme-ov-file#tips it's
now "strongly recommended" to have `rustfmt.toml` with and edition
configuration. This looks to be because `cargo fmt` reads the `edition`
field in `Cargo.toml`, but running `rustfmt` directly doesn't do that.
Editors often run `rustfmt` directly meaning they format differently
than `cargo fmt` without this configuration.

Author: alexcrichton

rustfmt.toml

@@ -0,0 +1 @@
+edition = '2024'