Create a rust-toolchain.toml file to fix the rust version for everyone

[bnjjj]

Following this PR #182 I had an issue caused by the new rust release (1.59) which detects more lints. But if you're still using 1.58.1 locally for example you will have other lint errors. I suggest to implement the same behavior we did on the router https://github.com/apollographql/router/pull/554