Allow for Hal Forms extensions following Chapter 7 of the spec

Hal-Forms describes extensibility of Hal Forms.

https://github.com/mamund/hal-forms/blob/master/index.asciidoc#7-extending-the-hal-forms-document

I would find it useful if it were possible to extend Hal Responses with custom attributes of https://github.com/mamund/hal-forms/blob/master/index.asciidoc#property-element. While it is possible to customize single existing Hal Form Property Attributes using the InputPayloadMetadata.customize function, it seems not possible to add additional attributes.

That would give implementers at least some of the flexibility Mike Amundsen has intended with chapter 7, e.g. to demonstrate a spring hateoas based working implementation of such an extension that could later become part of the spec.

Simple idea: a new annotation that allows additional key-value pairs on a field, sth like:

  @HalFormPropertyExtensions({
    @HalFormPropertyAttribute(name = "myProperty", value = "myValue"),
    @HalFormPropertyAttribute(name = "myProperty", value = "myValue")
  })
  private String schluessel;

Could probably also be achieved with @JsonAppend, now that I think about it. I'll let you know, and be it simply as a discussed extensibility of Spring Hateoas ;-)

Start with the InputPayloadMetadata.customize extension point and the Hal Forms property-element rules in the linked Chapter 7 specification. Compare the proposed annotation approach with JsonAppend and define how custom key-value attributes should be represented; done means an implementer can add extension attributes to Hal Forms properties.