Allow extending `HalFormsProperty` behind HAL-FORMS specification

We'd like to add custom attributes to HalFormsProperty. This seems allowed by HAL-FORMS specification but it seems currently impossible with Spring HATEAOS.

IMO, the best extension point would be somewhere here:

https://github.com/spring-projects/spring-hateoas/blob/3a9d0babc30b1aefada7d95e5d7f2676cc4d435c/src/main/java/org/springframework/hateoas/mediatype/hal/forms/HalFormsPropertyFactory.java#L83-L108

I guess that would mean adding a free Map to HalFormsProperty and a PropertyCustomizer SPI looking like this:

interface PropertyCustomizer {
  HalFormsProperty customize(HalFormsProperty property, InputPayloadMetadata payload, PropertyMetadata metadata, PropertyCreationContext context);
}