Redesign Attributes/Relationships around property wrappers

Exploration started here: https://github.com/mattpolzin/JSONAPI/tree/feature/transformed-property

This project is on hold until property wrapper chaining (or nesting) is supported (likely Swift 5.2).

The idea I am pursuing is to replace Attribute/TransformedAttribute with an @Attribute attribute and use an @Omittable attribute to represent when attributes can be omitted (i.e. the key is not required). I am a little less solid on relationships, but they would get similar treatment. Then, I additionally would like to flatten things so that attributes and relationships are defined in the same structure with their property attributes being the thing that differentiates them. Definitely have not worked out all of the details yet, but I hope that in addition to being a breaking change this will bring some concision and reduce the overall number of types needed a bit.

Something like

PersonProperties {
  // takes the place of current `Attribute`
  @Attribute
  var name: String

  // where age is stored as a `String` for some reason but we expect it to always be an `Int`
  // takes the place of current `TransformedAttribute`
  @Attribute(deserialize: StringToInt.self)
  var age: Int

  // takes the place of current `ToOneRelationship`
  @Relationship(to: City?.self)
  var hometown: Id?

  // takes the place of current `ToManyRelationship`
  @Relationship(to: [Person].self)
  var friends: [Id]
}

首先查看链接的 transformed-property 分支中的探索内容，以及现有的 Attribute、TransformedAttribute 和 relationship 类型。在定义 breaking change 的完成标准之前，先明确 property-wrapper 和 relationship 的设计，包括对链式调用或嵌套的支持。