Support subscriptions for expressions

[stephanpelikan]

Scenario

Some process-engines do not store business values and therefore do not evaluate expressions (e.g. of flows of exclusive gateways) on their own. They delegate this to the client instead.

Current Behaviour

Missing possibility to subscribe to expressions.

Wanted Behaviour

Add another command for subscribing to expressions to be evaluated:

data class SubscribeForExpressionsCmd(
  var restrictions: Map<String, String>,
  val processDefinitionKey: String,
  val payloadDescription: Set<String>,
  val action: (expression: String, payload: Map<String, Any>) -> Any
)

Expressions do not belong to task but to processes instead. Therefore the taskDefinitionKey changed to processDefinitionKey. Additionally the action takes the expression to be resolved and returns the result.