add cargo formatting check to CI

[alejandro-vaz]

ft. #396 @fereidani

implement a CI stage to ensure all committed code to the v2 branch is adequately formatted

cargo fit --all --check

it makes sure that all committed code is properly styled

this should be added to both the v1 and v2 branches

[alejandro-vaz]

this will break current project formatting a lot

one thing we should do in the future is have a clear STYLE.md file or similar

I'm not even sure I agree myself with all the style decisions cargo fmt takes

[emilio]

I don't like all the rustfmt decisions, but they're better than everybody doing their own. So +1 on enforcing fmt on CI for what is worth.