Allow extending `HalFormsProperty` behind HAL-FORMS specification

[reda-alaoui]

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:

spring-hateoas/src/main/java/org/springframework/hateoas/mediatype/hal/forms/HalFormsPropertyFactory.java

Lines 83 to 108 in 3a9d0ba

	return model.createProperties((payload, metadata) -> {
	String inputType = metadata.getInputType();
	HalFormsOptions options = optionsFactory.getOptions(payload, metadata);
	HalFormsProperty property = new HalFormsProperty()
	.withName(metadata.getName())
	.withRequired(metadata.isRequired()) //
	.withReadOnly(metadata.isReadOnly())
	.withMin(metadata.getMin())
	.withMax(metadata.getMax())
	.withMinLength(metadata.getMinLength())
	.withMaxLength(metadata.getMaxLength())
	.withRegex(lookupRegex(metadata)) //
	.withType(inputType) //
	.withValue(options != null ? options.getSelectedValue() : null) //
	.withOptions(options);
	Function<String, I18nedPropertyMetadata> factory = I18nedPropertyMetadata.factory(payload, property);
	return Optional.of(property)
	.map(it -> i18n(it, factory.apply("_placeholder"), it::withPlaceholder))
	.map(it -> i18n(it, factory.apply("_prompt"), it::withPrompt))
	.map(it -> model.hasHttpMethod(HttpMethod.PATCH) ? it.withRequired(false) : it)
	.orElse(property);
	});

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);
}

[odrotbohm]

It looks like this is related to GH-2275.