Split Rust sdk into a separate crate

[bigspider]

This PR splits the Rust sdk into its own separate folder exapp-sdk at the root of the repo.

Things that are still not satisfactory:

• Apps still need the lib folder and the create-lib.sh script, which the sdk also contains; it would be nice if all those were directly provided by the exapp-sdk.
• native app compilation is broken right now.
• CI is still broken

Improvements left for future PRs:

• Avoid using docker in the build process.
• The C sdk is also under app folder, and it would probably be better off in its own folder at the root of the repo; left for a future PR.
• The exapp-sdk crate could also provide some installable commands to extend cargo.