Field set (group of fields)

[tshemsedinov]

Metaschema supports nested structures, but for database we need to unroll grouped fields into plain structure.

Refs: metarhia/metaschema#291

Example:

({
  login: { type: 'string', unique: true, length: 30 },

  fullName: {
    givenName: { type: 'string', required: false },
    middleName: { type: 'string', required: false },
    surname: { type: 'string', required: false },
  },

  birth: {
    birthDate: 'string',
    birthPlace: 'string',
  },
});