Allow inherited DelegatingConverter to be processed

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Related to https://github.com/mapstruct/mapstruct-spring-extensions/issues/101

Hi,

I am trying to modify the annotation processor to support custom Converter interface but I don't know how to process DelegatingConverter by inheritance.

New code in `ConverterMapperProcessor`:
```java
delegatingConverterDescriptors =
annotations.stream()
.filter(ConverterMapperProcessor::isDelegatingConverterAnnotation)
.map(roundEnv::getElementsAnnotatedWith)
.flatMap(Set::stream)
.map(ExecutableElement.class::cast)
// Do not generate delegate without Mapper annotation on class
.filter(annotatedMethod -> annotatedMethod.getEnclosingElement().getAnnotationMirrors().stream().anyMatch(x -> x.getAnnotationType().toString().equals(MAPPER)))
.map(annotatedMethod -> new DelegatingConverterDescriptor(annotatedMethod, processingEnv))
.collect(toList());
```

Custom converter:
```java
public interface BaseMapper extends Converter {

@Nullable
T convert(@NonNull S source, @Context CycleAvoidingMappingContext context);

@Nullable
@Override
@Named("baseConvert")
default T convert(@NonNull S source) {
return convert(source, new CycleAvoidingMappingContext());
}

// Annotation is not processed
@DelegatingConverter
@InheritInverseConfiguration
default S invertConvert(T source) {
return invertConvert(source, new CycleAvoidingMappingContext());
}

S invertConvert(T source, @Context CycleAvoidingMappingContext context);
}
```

Mapper:
```java
@Mapper(config = MapperConfig.class)
public abstract class CarMapper implements BaseMapper {

// Required annotation to have delegate generated by processor
@DelegatingConverter
@Override
public Car invertConvert(CarDto source) {
return BaseMapper.super.invertConvert(source);
}
}
```

Delegate:
```java
@Component
public class CarDtoToCarConverter implements Converter {
private CarMapper delegateMapper;

public CarDtoToCarConverter(@Autowired final CarMapper delegateMapper) {
this.delegateMapper = delegateMapper;
}

@Override
public Car convert(final CarDto source) {
return delegateMapper.invertConvert(source);
}
}
```

Wanted mapper:
```java
@Mapper(config = MapperConfig.class)
public abstract class CarMapper implements BaseMapper {
// Nothing to override
}
```

Thanks

Start in ConverterMapperProcessor and trace how @DelegatingConverter methods are collected from roundEnv; compare that path with the inherited BaseMapper example. Done means the annotation on BaseMapper.invertConvert is recognized for CarMapper without an override and the corresponding delegate is generated.

Written by the indexing model from the issue text.