Improve API when cheater-detection is disabled

[conradoplg]

When cheater detection disabled, the Coordinator doesn't need to pass PublicKeyPackage::verifying_shares since it's not used anymore. But they still need to choose identifiers to use in the signature_shares BTreeMap argument.

Ideally, when cheater-detection is disabled, this argument could become a BTreeSet, though I feel it's awkward to have a function have different arguments depending on a feature.

Alternativelly we could add another function, e.g. aggregate_without_cheater_detection (and then we could deprecate the cheater-detection feature)