Add manual interaction to state machines

[paullatzelsperger]

Feature Request

Our state machines should have a way for humans to manually advance the state. For instance, this will be needed when manual sign-off on a contract negotiation is required.

Which Areas Would Be Affected?

core: transfer and negotiation state machine

Why Is the Feature Desired?

In certain scenarios human intervention will be required, for example signing off on contract negotiations.

Solution Proposal

To provide maximum flexibility, the implementation of this feature should be able to dynamically make a decision (eg. through a Function) whether or not an entity's state can be modified automatically. In combination with adding an extension point here, implementors can easily add their own logic.
For example, one use case might only want to add manual interaction to a certain set of states, but might also want to decide based on a certain property of an asset.