Add rust-toolchain file

[cryptoquick]

Summary of changes

Changes introduced in this pull request:

• Adds rust-toolchain file pointing to rust 1.52.1
  • We're just using a new Rust string method, it's mostly just so our code is cleaner and modern, is all

Reference issue to close (if applicable)

Closes #1131

Other information and links

In case anyone's curious:

• split_once was added in 1.52: https://doc.rust-lang.org/std/primitive.str.html#method.split_once
• It's used here:

  forest/node/rpc-client/src/client.rs

  Line 120 in 6f9edae

  let mut http_res = match &api_info.split_once(':') {

Also note, our CI is already configured to use 1.52.1

[cryptoquick]

Also, to review this, just use cargo check, it will result in slightly different behavior as it downloads the toolchain specific to this project.