[Feature Proposal] Allow users to extend default options

[wenderjean]

Description

• We should allow users to extend defaults and set generic options for their own Structure instance.

Use cases

• Empty strings: The default behavior for Strings is not to allow empty values, we can set empty: true for each domain but it would be great if we can do a generic configuration.
• It can also prepare the codebase for this feature.

Example

const MyStructure = Structure.extends({
  validations: [
    {
      type: String,
      empty: true
    }
  ]
});

module.exports = MyStructure;

cc/ @talyssonoc

[talyssonoc]

Also, it would be a great fit to support #12.

Other use cases that could be added is having different defaults for each use of Structure in the app, for example:

• A base ValidationStructure that would have some set of preset validations that would be used only to validate the body of HTTP requests;
• A base EntityStructure (and/or AggregateStructure) that would always have an id field of the type Number.

As a suggestion, we could have an internal "structure provider" that receives the attributes and return a new custom Structure lib, and the default one (that is returned when calling `require('structure')) would be just this provider called with a set of default values and the user would extend from that. The idea would be that every custom Structure can be extended to generate a new one inheriting the settings from the parent one, so we could do this:

const MyStructure = Structure.extends({
  validations: [
    {
      type: String,
      empty: true
    }
  ]
});

const AnotherStructure = MyStructure.extends({
  fields: [
    {
      type: Number,
      name: 'id'
    }
  ]
});

[wenderjean]

@talyssonoc We can also add this one as related using the fields approach.

We need the plugin system yet but using field options we could have the defaults timestamp.

[talyssonoc]

Yep, it does make sense!