Setup a `rust-toolchain.toml` file to specify correct Rust toolchain version

Summary of the new feature / enhancement

Coming out of https://github.com/PowerShell/DSC/pull/1422 we discovered our build was using ms-stable which had the unintended side effect of pinning us to Rust 1.88. You'll note our use of msrustup (when available), it is Microsoft's tool for installing compliant internal Rust toolchains. This is used in our release pipeline, and can be used by Microsoft developers.

In June 2024, the ms-stable moniker (passed to msrustup) was retired and we were supposed to move to the moniker ms-prod to get updates. We didn't do that, so it has stayed at the equivalent of ms-prod-1.88. At the same time, it is highly recommended to take updates intentionally, hence that PR does a blanket switch from ms-stable to ms-prod-1.93.

It would be best for us to specify and re-use this value in one place. The most correct and canonical way to do that is with a rust-toolchain.toml file. However, we need to do this in a way that when msrustup isn't available (such as for community devs) everything continues to work without a hitch.

Proposed technical implementation details (optional)

No response

Start by reviewing PR 1422 and the release pipeline references to msrustup, including the current ms-stable or ms-prod-1.93 selection. Then determine how rust-toolchain.toml should provide one canonical version while preserving builds for developers without msrustup. Done means the toolchain is specified once and both release and community builds continue to work.