Aggregate specified Assertions

**Enhancement**

Use case:
- I have generated the assertions for my model
- I have created some custom assertions
- I'm still using the standard assertions

And I'd like to use a single "Assertions" class (entry point).

Configuration could be:
- the FQN of the aggregation class
- a list of packages containing Assert implementations
- a list of classes implementing Assert
- a list of includes to restrict classes to aggregate (regex)
- a list of excludes to exclude classes to aggregate (regex)

So basically the generator will copy all the model generated and custom

```
public static XxxxxAssert assertThat(Xxxxx actual) {
return new XxxxxAssert(actual);
}
```

into one class inheriting Assertions.