CR — CaseTask.CaseRef should allow for expressions to support dynamic subcases

There are several use cases with my current client where we do not know in advance what will be the actual case that is going to be invoked as a CaseTask. Broadly, the main case has 2 phases, each lasting for about 6 months. The second phase is implemented mostly by means of a subcase (plus some other things). We only know which specific subcase will be invoked right before the phase starts.
However, in the current CMMN spec, we must know the precise subcase at the time of modeling (through the caseref property). We need a mechanism to have some logic determine the subcase at the moment it becomes Active (i.e., not when it becomes Available). Preferably through some sort of expression.
Also at my previous employer we ran into various situations where we had a standard main case model (in COTS software) and then per customer implementation we could vary the specific subcases and subprocesses to be invoked, by means of an expression that would look into e.g. a lookup table.

Note: when we add this to the spec, we need obviously a fixed input/output contract to which the subcase must adhere in order to be able to do parameter binding.

Add "caseRefEpression" to CaseTask and "processRefExpression" to ProcessTask

Make the selection of a specific Case or Process task an expression so that a process or case can be selected at runtime and doesn't have to be specified at design-time.