support VariableAware delegates and listeners

[jangalinski]

Scenario

with the introduction of VariableReader and VariableWriter in 1.0.0 ... let's go one step further. Why should we use the default camunda delegate/listener API, when we could directly do:

@Bean
  fun writeOrderTotal() = VariableAwareExecutionListener { execution ->
    val total = execution.get(ORDER_TOTAL)
    execution.set(ORDER_TOTAL, total)
  }

Current Behaviour

VARIABLE.on(execution).set(value)

Wanted Behaviour

execution.set(VARIABLE, value)

[zambrovski]

Wht is the main motivation to creatge an own API instead of using existing one?
Is there something I can do better with it? Can you provide some insights please?

Contra: I'll have to learn and educate an additional API (instead of a de-facto standard one introduced by Camunda), make all tools work with it (testing, mocking, etc...)

[zambrovski]

By the way - this API change is relevant for Java only, so no need to do it for kotlin this way.