Flowable-rest module add/delete multiple instances execution API does not exist

**Is your feature request related to a problem? Please describe.**

Flowable-rest module add/delete multiple instances execution API

In flowable-engine Include **runtimeService.addMultiInstanceExecution** and **runtimeService.deleteMultiInstanceExecution** function.

```
public Execution addMultiInstanceExecution(String activityId, String parentExecutionId, Map executionVariables) {
return (Execution)this.commandExecutor.execute(new AddMultiInstanceExecutionCmd(activityId, parentExecutionId, executionVariables));
}

public void deleteMultiInstanceExecution(String executionId, boolean executionIsCompleted) {
this.commandExecutor.execute(new DeleteMultiInstanceExecutionCmd(executionId, executionIsCompleted));
}
```

Hope to receive support in flowable-rest module.

Inizia nel modulo Flowable-rest e confronta i suoi endpoint runtime-service esistenti con le funzioni richieste addMultiInstanceExecution e deleteMultiInstanceExecution. Traccia il modo in cui queste operazioni di runtimeService vengono esposte tramite REST, quindi aggiungi la copertura per entrambe le operazioni e verifica che la REST API supporti la creazione e l’eliminazione di esecuzioni multi-istanza.

Scritto dal modello di indicizzazione a partire dal testo della issue.