New Feature: "Contracts" => enable/disable contract integration tests

[jonas089]

This PR introduces a change that helps satisfy rust-analyzer when cloning the spectre repo and opening it in an IDE.
I introduce a dummy crate with a readme pointing to the real contracts crate and a feature that makes the dependency on contract integration tests optional.

I believe that this is an improvement to the workspace since we can't push the compiled binaries to GitHub and we want developers to be able to isolate and test / work on the ZK logic without having to bother setting up Smart Contracts / pulling in the external crate.

@nulltea agrees to my proposal which is why I have decided to create this PR. Please comment for suggestions and further improvements :).

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