Support the "List/Edit Custom Fields" endpoints

[JOJ0]

If we want, we could implement a method and classes for fetching/listing the custom fields so you'd at least have access to them and can find out the custom field name by using the field_id from above

• https://www.discogs.com/developers#page:user-collection,header:user-collection-list-custom-fields
• https://www.discogs.com/developers#page:user-collection,header:user-collection-edit-fields-instance

For example something like

custom_fields = me.collection_fields() # calls /api/{username}/collection/fields
for note in instance.notes:
   print(custom_fields.find_by_id(note.field_id).name)    

pseudo code, might not work 😛

Originally posted by @AnssiAhola in #166 (reply in thread)