ci: Add workflow to check for uncommitted codegen changes

[svix-jplatte]

Tested successfully: https://github.com/svix/svix-webhooks/actions/runs/13500955055/job/37718984950

[svix-mman]

@svix-jplatte
Is the plan here to eventually check for uncommitted changes on all of the languages. Or is this workflow only for rust?

[svix-jplatte]

This workflow checks for any changes done by regen_openapi.sh.
The plan is for that script to regenerate all of the libs (again).

[svix-mman]

Is the long term plan is not to track the generated libs in git, Or we will regen in CI so we can verify that committed libs are up to date?

[svix-jplatte]

The long term plan is to continue tracking the generated libs in git, and have CI verify that any changes to the spec or templates are reflected in the libs / no manual changes were made to libs that the script would overwrite again. This PR adds that CI check, though it only affects Rust for now because that's the only lang the regen script affects that also has its source committed. Makes sense?