List types in CustomFields

[michaelbromley]

Is your feature request related to a problem? Please describe.
Currently the permitted data types for a custom field are:

export type CustomFieldType = 'string' | 'localeString' | 'int' | 'float' | 'boolean' | 'datetime';

It would be useful to allow lists/arrays of each of the above. This is already being implemented as part of #414 and a similar approach could be used for custom fields.

Describe the solution you'd like
Add a list: boolean config property to the TypedCustomFieldConfig type, so a list can be defined as:

customFields: {
  Product: [
    {
      name: 'keywords',
      type: 'localeString',
      list: true,
    },
 ],
}

In the AdminUI, this would then be presented as the regular string input but contained in a "list container" which includes a means to add new items, delete existing items, and re-order the list.

In the DB they would be stored as a simple-array type.