Add rust-toolchain.toml

[Shourya742]

We have observed instances where pull requests include unexpected formatting changes and Clippy fixes that were not present initially. This issue occurs because contributors are using different Rust versions, leading to inconsistencies caused by differences in toolchain behavior. To resolve this, we should add a rust-toolchain.toml file to the repository to enforce a consistent Rust version across all contributors. This will ensure that formatting and linting remain uniform, preventing unrelated changes from appearing in pull requests.

Few recent examples:

1. fix sigterm issue with translator sv2 #1319 (review)

[Fi3]

yep I agree, but we removed it cause it was creating issues to some IDEs don't know which ones so before re adding it we should double check it.

[plebhash]

@Fi3 we were discussing this in a call earlier today

one of the questions that came up was why we're using nightly on the fmt workflow:

stratum/.github/workflows/fmt.yaml

Line 30 in 7483090

toolchain: nightly

@jbesraa mentioned that you had explained this a while ago over Discord but we couldn't find the messages

[Fi3]

last time that I looked into it was required by our rustfmt.toml that if I remember correctly have been added by @rrybarczyk so maybe she know better then me