Additionally, there are two other possible WitnessScopes: CustomContracts (i.e. the caller can specify which contracts the signature is effective for) and CustomGroups (used for identifying a set of mutually trusted contracts - for example, all the contracts of a single dApp published by a single party). We can open a separate issue for tracking CustomContracts and CustomGroups support, but it would be great if we added at least CustomContracts support as part of the work to add support for None, Global and CalledByEntry
See #89