Add a rust-toolchain file

[Subsegment]

Add a rust-toolchain file to better guide compilation with the corresponding rust version

Like this:

[toolchain]
# The toolchain used to compile the code
channel = "nightly-2024-01-12"

[robinsdan]

It compiles on stable and nightly. why need this? I think the only usecases for a toolchain file are:

• the code uses some nightly features and can not compile on stable, then set toolchain="nightly";
• the code uses some very unstable nightly features and can only compile on a old nightly version, then set toolchain="nightly-xxxxx"