Add support for Optional-wrapping property builders

[octylFractal]

I think it is worthwhile to allow Optional for properties that use property builders, so I wrote code to support such an option.

The value is an empty Optional by default, and uses Optional.of(builder.build()) to generate an Optional from the builder.

Ex:

@AutoValue
abstract class Foo {
  @AutoValue.Builder
  interface Builder {
    Nested.Builder nestedBuilder();
  }
  
  abstract Optional<Nested> nested();
}

@AutoValue
abstract class Nested {
  
  static Builder builder() {
    return new AutoValue_Nested.Builder();
  }
  
  @AutoValue.Builder
  interface Builder {
  }
}

[octylFractal]

I've added the tests here as well, @eamonnmcmanus.