forc call support empty `--external-contracts` flag

Currently, there is no way to explicitly pass in empty external contracts.

Context

This causes the forc call functionality to always run determine_missing_contracts (which makes RPC calls). In some scenarios, users may not want to explicitly make RPC calls (reduce latency) and/or may already know that the contract does not integrate with any other contracts.

Hence, forc-call CLI should allow one to explicitly specify no external contract calls are needed.