Merged PR 4228: First version of sales - monthly trends

# Description

New model that aggregates in a monthly basis the information from Xero sales. The aggregation is done by several categories, most of them to be included as filters in PBI. Metrics represent the amount without taxes in: 1) the current month, 2) the previous month, 3) twelve months ago, 4) current year YTD (fiscal year) and 5) previous year YTD (fiscal year).

It also propagates the model to reporting

# 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.
- [ ] I've picked the right materialization for the affected models. **I was hesitating between table or view. Table should speed up tests, but I intend to create a reporting table with a similar structure so probably a bit of waste of space. Anyway, not a big deal now**

# Other

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

Related work items: #26618

models/reporting/xero/xero__sales_monthly_trends.sql

@@ -0,0 +1,20 @@
+with
+    int_xero__sales_monthly_trends as (
+        select * from {{ ref("int_xero__sales_monthly_trends") }}
+    )
+select
+    document_issued_month_utc as document_issued_month_utc,
+    fiscal_year as fiscal_year,
+    previous_fiscal_year as previous_fiscal_year,
+    document_status as document_status,
+    document_type as document_type,
+    id_deal as id_deal,
+    accounting_financial_l1_aggregation as accounting_financial_l1_aggregation,
+    accounting_financial_l2_aggregation as accounting_financial_l2_aggregation,
+    accounting_financial_l3_aggregation as accounting_financial_l3_aggregation,
+    amount_wo_taxes_in_gbp as amount_wo_taxes_in_gbp,
+    previous_month_amount_wo_taxes_in_gbp as previous_month_amount_wo_taxes_in_gbp,
+    previous_year_amount_wo_taxes_in_gbp as previous_year_amount_wo_taxes_in_gbp,
+    ytd_amount_wo_taxes_in_gbp as ytd_amount_wo_taxes_in_gbp,
+    previous_year_ytd_amount_wo_taxes_in_gbp as previous_year_ytd_amount_wo_taxes_in_gbp
+from int_xero__sales_monthly_trends