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/schema.yml

@@ -1395,3 +1395,98 @@ models:
         data_type: character varying
         description: |
           The code and name of the account separated by a "-".
+
+  - name: xero__sales_monthly_trends
+    description: |
+      This model provides monthly aggregated sales and credit note data with 
+      fiscal year alignment and period-over-period comparisons. 
+
+      The model is built by aggregating the `int_xero__sales_denom_mart` model 
+      at the monthly level, computing sales amounts without taxes in GBP, and 
+      incorporating key financial aggregations. 
+
+      Additionally, it calculates prior period trends:
+        - Previous month sales
+        - Previous year (same month) sales
+        - Year-to-date (YTD) sales for the current fiscal year
+        - YTD sales for the previous fiscal year
+
+      The fiscal year is determined based on an April-March cycle, where a 
+      fiscal year is labeled by the year in which it ends (e.g., FY 2025 
+      represents April 2024 – March 2025).
+
+    columns:
+      - name: document_issued_month_utc
+        data_type: date
+        description: "The first day of the month when the document was issued, truncated to the month level."
+        data_tests:
+          - not_null
+
+      - name: fiscal_year
+        data_type: integer
+        description: |
+          "The fiscal year in which the document's issued month falls. 
+          The fiscal year runs from April to March and is labeled by the year in which it ends 
+          (e.g., FY 2025 represents April 2024 – March 2025)."
+        data_tests:
+          - not_null
+
+      - name: previous_fiscal_year
+        data_type: integer
+        description: "The fiscal year prior to the current one, based on the April-March cycle."
+        data_tests:
+          - not_null
+
+      - name: document_status
+        data_type: text
+        description: "The status of the document (e.g., PAID, VOIDED, AUTHORISED, etc.)."
+        data_tests:
+          - not_null
+
+      - name: document_type
+        data_type: text
+        description: "The type of document, indicating whether it is an invoice or credit note."
+        data_tests:
+          - not_null
+
+      - name: id_deal
+        data_type: text
+        description: "The deal associated with the transaction, if applicable. Defaults to 'UNSET' if not provided."
+        data_tests:
+          - not_null
+
+      - name: accounting_financial_l1_aggregation
+        data_type: text
+        description: "The Level 1 aggregation for Financial reporting."
+
+      - name: accounting_financial_l2_aggregation
+        data_type: text
+        description: "The Level 2 aggregation for Financial reporting."
+
+      - name: accounting_financial_l3_aggregation
+        data_type: text
+        description: "The Level 3 aggregation for Financial reporting."
+
+      - name: amount_wo_taxes_in_gbp
+        data_type: numeric
+        description: "Total sales amount without taxes, converted to GBP, aggregated at the monthly level."
+
+      - name: previous_month_amount_wo_taxes_in_gbp
+        data_type: numeric
+        description: "The sales amount without taxes (GBP) for the previous month within the same category."
+
+      - name: previous_year_amount_wo_taxes_in_gbp
+        data_type: numeric
+        description: "The sales amount without taxes (GBP) for the same month in the previous year within the same category."
+
+      - name: ytd_amount_wo_taxes_in_gbp
+        data_type: numeric
+        description: |
+          "Year-to-date (YTD) total of sales amount without taxes (GBP) from the start of the current fiscal 
+          year up to the current month."
+
+      - name: previous_year_ytd_amount_wo_taxes_in_gbp
+        data_type: numeric
+        description: |
+          "Year-to-date (YTD) total of sales amount without taxes (GBP) from the start of the previous fiscal 
+          year up to the same month."