clientlib: Add zkproof generation

[arnaucube]

resolves #16 , resolves #9

Overview:

• contracts:
  • Connect the zkVerifier with the AnonVoting contract on vote method
  • Removed the zkVerifier4lvls & 10lvls, leaving only the zkVerifier16lvls, which should allow for 65k users in the census, and the proving time is still quite fast, so no need to add complexity with multiple circuits for different census sizes.
  • Added contract tests with real zkproof being verified in the contract
• clientlib:
  • generate zkproof
  • castVote method (to send the vote+zkproof to the contract)
• other:
  • add circuit16.wasm & circuit16.zkey files needed to generate the zkproofs (these files are from https://github.com/aragonzkresearch/anon-voting-demo/issues/14#issuecomment-1424007982 , and related to the specific zkVerifier16lvls.sol that we have for testing)

[arnaucube]

Update: rebased (& solved git conflicts) this PR branch (feature/zkproof) to last main changes after https://github.com/aragonzkresearch/anon-voting-demo/pull/26 has been merged into main.