Create Custom Field Definition

**This endpoint creates a new custom field definition.** Custom field definitions are created with the given `name` and `field_type`. Although field names are stored in a case-sensitive manner, all field names must be case-insensitively unique. This means you may create a field named `CamelCase` or `camelcase`, but not both. Additionally, a Custom Field name cannot collide with any Reserved Field names. You should save the returned `id` value in order to update or delete the field at a later...