feat: add models and module for CMS collections

[EmilePerron]

At the moment, the API client offers models and a client module for the "meta" models (AuthorizedUser, Authorization and Application), but it does not yet include functionality for the CMS collection endpoints and models.

• Models
  • Add Collection model
  • Add CollectionItem model
  • Add Field model
  • Add FieldType model
• Module
  • List Collections (GET)
  • Get Collection (GET)
  • List Collection Items (GET)
  • Remove Collection Items (DELETE)
  • Create Collection Item (POST)
  • Publish Collection Items (PUT)
  • Update Collection Item (PUT) cancelled: only one item update method is needed, and PATCH was used.
  • Patch Collection Item (PATCH)
  • Get Collection Item (GET)
  • Remove Collection Item (DELETE)

Webflow API documentation: https://developers.webflow.com/reference
Models documentation: https://developers.webflow.com/docs/models

[EmilePerron]

FieldType is actually implemented as an Enum, and a Validation model has also been added.