Add required dependency flag to signature selectors

[NDStrahilevitz]

Requirement

Signature event selectors currently serve de-facto as a "event dependency" mechanism similar to the one in the event defintions. There is a need to allow signatures to define selectors which do not automatically load the selected event (for example signatures which consume various optional signatures).

Solution

Event dependencies have a required flag which indicates wether tracee should fail when the event is not available. A similar flag can be used here. In the context of signatures this will indicate that the signature can function without input from the selected event.