Support subscriptions for expressions

### 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:

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