Refactor and introduce new prover endpoints

- Refactor the prover to compute the proving key commitment when booting
(previously recomputing on every `Prove` call)
- Introduce a `Verify` endpoint to verify a ZKP generated via the
`Prove` endpoint.
- Introduce a `GenerateContract` endpoint to generate a solidity
contract for the prover circuit.
- Upgrade from 8 to 16 max validators (testnet)
- Updade gnark fork to fix bn254 proof serialization (currently broken
on gnark)
- Introduce `unionpd example-verify` command to execute a prove/verify
call
- Introduce `unionpd gen-contract [path]` command to dump the solidity
contract