Define interface for custom signee logic

[HauklandJ]

Part of the Parallell signing epic.

App developers should be able to implement an interface for the purpose of calculating the signees dynamically.

We plan to have a config property on the singing task in bpmn.xml where the app developer specifies which implementation of the interface should be used for that task. An ID-prop on the interface will be used to identify the correct implementation.

The interface implementation can for instance look at the datamodel from a previous task and extract relevant info to determine the signees.

pseudo:

string SigneeLogicId
List<Signee> GetSignees(Instance instance, string TaskId)