refactor: improve initial project setup

[gavriliumircea]

This PR aims to implement all the changes discussed with @razgraf.

[razgraf]

We'll probably need the hex representation of both the root and the proof instead of u8 arrays. Apart from api/eligibility and api/campaign should we also return them within api/upload?

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I'm currently playing with the eligibility route and added two extra items to the response so I can hopefully use them when interacting with the contracts. Using the hex crate...

let response_json = &EligibilityResponse {
     index: recipient_index,
     proof: serialized_proof,
     proof_hex: proof.lemma().iter().map(|x| hex::encode(x)).collect(),
     root_hex: hex::encode(root),
};