Merged PR 5047: First API unified version of Verifications

# Description

Creates a first unified version of API verifications. This is a very simple model as it aims to capture the minimal essence of Verifications in a unified view. Any very-in-depth-API-specific detail is not available here.

Additionally, I propagated the Check-in date in the Athena model.

This is needed for API Bookings KPIs future models.

# Checklist

- [X] The edited models and dependants run properly with production data.
- [X] The edited models are sufficiently documented.
- [X] The edited models contain PK tests, and I've ran and passed them.
- [X] I have checked for DRY opportunities with other models and docs.
- [X] I've picked the right materialization for the affected models. **Currently as a view, despite it's 70k records. If this grows we could consider other materialisations**

# Other

- [ ] Check if a full-refresh is required after this PR is merged.

Related work items: #29374

models/intermediate/athena/schema.yml

@@ -248,6 +248,12 @@ models:
         data_tests:
           - not_null
 
+      - name: checkin_date_utc
+        data_type: date
+        description: "Date of checkin for the booking"
+        data_tests:
+          - not_null
+
       - name: checkout_date_utc
         data_type: date
         description: "Date of checkout for the booking"