Improve dependency management with Stainless

[yannbolliger]

Currently, for each change to the rust-interop branch of Stainless, we need to do a prerelease with a special tag to depend on that specific version of Stainless. The pre-release needs to have the binary attached that is then downloaded by our CI and used.

There could be less labour-intensive ways to achieve the same:

• Move away from using rust-interop releases, but rather https://jitpack.io/#epfl-lara/stainless/
• only specify a commit hash as dependency
• add a script that can download the correct backend version for developers