Feature/derived state

[codingnuclei]

Related to: #688

This is a RFC PR showcasing a possible enhancement to entities.

It enables an entity schema to define attributes which are derived.
It follows similar patterns already used by this library.

One thing to note is that the derived value is evaluated after all the other attributes have been formatted.

Usage:

const rating = (store: any) => {
  return 'some calculated value'
};

export const Store = new Entity({
  name: 'Store',
  attributes: {
    // .... removed for brevity
    oneStars: { type: 'number' },
    twoStars: { type: 'number' },
    threeStars: { type: 'number' },
    fourStars: { type: 'number' },
    fiveStars: { type: 'number' },
    rating: {
      derive: rating,
    }
  },
  table: MyTable,
} as const);

Future considerations:

• I have not looked at testing/docs/types in this RFC but based on feedback will make this a full fledge PR ready for merging.
• derived state which depends on other derived state
• ignore derived state fields in put/update request