Add zksync as a compilation target

[mmv08]

Context / issue

With the zksync's zkEVM release on the horizon, we'd like to optionally support it as a compilation target to enable a possible future deployment.

Proposed solution

Use ZkSync's hardhat plugins that can be used optionally (perhaps as an env variable)

Additional context

I experimented a little bit with zksync's compiler, and it's available here: https://github.com/safe-global/safe-contracts/tree/zksync. Our contracts required some adjustments making it impossible to compile production-ready audited contracts, but with changes in #462 it should be possible

[mmv08]

closed in favour of #766