Consider adding a `rust-toolchain.toml` file

[thunderbiscuit]

The project currently does not include a rust-toolchain.toml file file. Adding one would help ensure that contributors, CI pipelines, and users build with the same Rust version and toolchain configuration.

I notice we have a rust-version file that I assume is performing a similar task, but after looking into it seems that this is not as standard nor as feature-rich as the rust-toolchain.toml file. I don't know if I somehow need to provide it as an argument somewhere in my build, but the file also appears not to have any effect on my local machine (when reviewing #310 my local 1.85.1 toolchain threw an error because of a nightly feature, and updating manually to the 1.89 version fixed the build, but in other words my local build workflow doesn't pick up on this rust-version file at all). On the other hand, adding a simple rust-toolchain.toml fixed my issue instantly.

I assume doing this would break the CI in places so I'll open a PR to test.

Edit: Example PR here: