Feature request: Custom extended type support

[alicederyn]

Current thinking is to support custom "single-property builder" templates, with the API cloned onto the enclosing builder. For instance, a Set template might look like:

public class SetPropertyTemplate<T>
    extends AbstractPropertyTemplate<Set<T>, SetPropertyTemplate<T>> {
  private final HashSet<T> property = new HashSet<>();
  public void addProperty(T element) {
    property.add(checkNotNull(element));
  }
  public void addAllProperty(Iterable<T> elements) {
    for (T element : elements) {
      addProperty(element);
    }
  }
  public void clearProperty() {
    property.clear();
  }
  public Set<T> getProperty() {
    return unmodifiableSet(property);
  }
  @Override public void mergeFrom(Set<T> elements) {
    addAllProperty(elements);
  }
  @Override public ImmutableSet<T> getImmutableValue() {
    return ImmutableSet.copyOf(property);
  }
  @Override public void clear() {
    property.clear();
  }
}

Here, "Property" is a magic string that gets replaced by the actual property name when the API is cloned, so for instance a set property called Values would result in methods addValues, addAllValues, clearValues and getValues. It should also be possible to inline the template definition as part of code generation, making custom extensions indistinguishable from built-in extensions. We could even use templates for all existing customization.

A key motivating example is values like IDs and timestamps that are generally minted at build time, but should not be overridden during deserialization.