[FEATURE] Support for factory methods

[sureshcentral]

@factory creates a factory method that can create different implementations:

Usage:

@Factory(HexagonImpl.class, TrianglImple.class)
interface Shape {
  ...
}

// Elsewhere ...
public class HexagonImpl implements Shape { ... }
public class TriangleImpl implements Shape { ... }

// Usage:
// Lombok creates a hidden 'factory' object in the interface Shape.
// The factory object returns a specific implementation.
Shape s = Shape.factory.create(Shape.Type.HEXAGON, ...params);

This is mostly useful for the so-called "sealed factories", where the
implementations types are known at compile time.

Implementation:

Shape.java:

// @Factory({TriangleImpl.class, HexagonImpl.class})
public interface Shape {
  /* Business methods. */
  int computeArea();

  /* ========= Generated by Lombok ========= */
  enum Type {TRIANGLE, HEXAGON}; // Enum generated by stripping out 'Impl' from the specified impl classes in the @Factory annotation.
  static final ShapeFactory factory = ShapeFactory.instance;
  class ShapeFactory {
    private static final ShapeFactory instance = new ShapeFactory();
    private ShapeFactory() {}
    public Shape create(Shape.Type type, Map<String, ?> properties) {
      switch (type) {
        case TRIANGLE:
          return new TriangleImpl(properties);
        case HEXAGON:
          return new HexagonImpl(properties);
      }
      throw new IllegalArgumentException("Unknown shape type: " + type);
    }
  }
  /* ========= End generated ========= */
}

TriangleImpl.java:

class TriangleImpl implements Shape {

  TriangleImpl(Map<String,?> properties) {
  }

  @Override
  public int computeArea() {
    // TODO
    return 0;
  }
}

HexagonImpl.java:

class HexagonImpl implements Shape {
  HexagonImpl(Map<String,?> properties) {
  }

  @Override
  public int computeArea() {
    // TODO
    return 0;
  }
}

Usage:

import Shape;

public class FactoryExample {
  public static void main() {
    Shape hex = Shape.factory.create(Shape.Type.HEXAGON, null);
  }
}

[rzwitserloot]

This doesn't strike me as boilerplate. Or rather, it strikes me as a feature that isn't going to have any users except 1 (you).