Merged PR 2232: KPI refactor - 1st step, bookings

First step on refactor of kpis:
- Remove relative incremental vs. previous year computation from the source model (`mtd_booking_metrics`, in this case)
- Aggregate the source mtd global metrics models into a single model: `int_mtd_vs_previous_year_metrics` (to enable multi-source weighted metric computation) and compute previous year value and relative increment. Now this logic is encapsulated into a macro `calculate_safe_relative_increment`, easing readability and providing a bit more robustness.
- End-to-end continuity to not break the existing dashboard display in `int_core__mtd_aggregated_metrics`

This is a substep of the global change. All info can be found in the documentation [here](https://www.notion.so/knowyourguest-superhog/Refactoring-Business-KPIs-5deb6aadddb34884ae90339402ac16e3)

Related work items: #18202

models/intermediate/cross/int_mtd_vs_previous_year_metrics.sql

@@ -0,0 +1,50 @@
+/*
+This model pivots the data of the different mtd metrics models to get 
+previous year for each line & computing relative increment.  --
+*/
+
+{{ config(materialized="table", unique_key="date") }}
+with
+    int_core__mtd_booking_metrics as (
+        select * from {{ ref("int_core__mtd_booking_metrics") }}
+    ),
+    int_dates_mtd as (select * from {{ ref("int_dates_mtd") }}),
+
+    plain_kpi_combination as (
+        select
+            d.year,
+            d.month,
+            d.day,
+            d.is_end_of_month,
+            d.is_current_month,
+            d.date,
+            bookings.created_bookings,
+            bookings.check_out_bookings,
+            bookings.cancelled_bookings
+        from int_dates_mtd d
+        left join int_core__mtd_booking_metrics bookings 
+            on d.date = bookings.date
+    )
+select
+    current.year,
+    current.month,
+    current.day,
+    current.is_end_of_month,
+    current.is_current_month,
+    current.date,
+    previous_year.date as previous_year_date,
+
+    -- BOOKINGS --
+    {{ calculate_safe_relative_increment('created_bookings') }},
+    {{ calculate_safe_relative_increment('check_out_bookings') }},
+    {{ calculate_safe_relative_increment('cancelled_bookings') }}
+
+from plain_kpi_combination current
+left join plain_kpi_combination previous_year
+    on current.month = previous_year.month
+    and current.year = previous_year.year + 1
+where
+    (
+        current.is_end_of_month = 1
+        or (current.is_current_month = 1 and current.day = previous_year.day)
+    )