feat: add type safe Ad model

src/kleinanzeigen_bot/resources/ad_fields.yaml

@@ -1,24 +0,0 @@
-active:  # one of: true, false
-type:  # one of: OFFER, WANTED
-title:
-description:
-category:
-special_attributes: {}
-price:
-price_type:  # one of: FIXED, NEGOTIABLE, GIVE_AWAY, NOT_APPLICABLE
-shipping_type:  # one of: PICKUP, SHIPPING, NOT_APPLICABLE
-shipping_costs:
-shipping_options: []  # see README.md for more information
-sell_directly:  # requires shipping_options to take effect
-images: []
-
-contact:
-  name:
-  street:
-  zipcode:
-  phone:
-
-republication_interval:
-id:
-created_on:
-updated_on: