[WIP] Add Support for pre-defined Custom Field Types

[westonganger]

Solves feature request within https://github.com/locomotivecms/engine/issues/1277#issuecomment-482546151

Custom field types are defined as .json files within config/custom_field_types/

Example Usage:

// config/custom_field_types/color_selector.json

{
  "type": "select",
  "options": [
    {"label": "Pink - #e69798","value": "bg-pink"},
    {"label": "Yellow - #f3c577", "value": "bg-yellow"}
  ]
}

// within the section definition

"settings": [
  {
    "type": "color_selector",
    "id": "background_color",
    "label": "Background Color"
  }
]

TBD:

• Deploy fails for Content Type which utilize a custom field type
  • Resource invalid: entries_custom_fields is invalid, #5d00127ee1382349794fe075: Type is not included in the list
• Section & Site Metafields editors fail to load when custom field type is used

I will be needing some assistance to complete this feature as so far I am only familiar with the Filesystem adapters.

Please feel free to refractor as necessary.

Related PR's:

• Wagon: https://github.com/locomotivecms/wagon/pull/375
• Engine: https://github.com/locomotivecms/engine/pull/1336

[westonganger]

Please place this functionality on high priority. This is much needed.

[did]

@westonganger let's talk about it on Gitter!