getBy method
Return the StudyProtocol with the specified protocolId,
versionTag is the tag of the specific version of the protocol to return.
The latest version is returned when not specified.
Implementation
@override
Future<StudyProtocol> getBy(String protocolId, [String? versionTag]) async =>
StudyProtocol.fromJson(await _rpc(GetBy(protocolId, versionTag)));