Merged PR 4616: Adds first day month for reporting purposes

# Description

Just adds the first day of the month for AM models. The problem is that the display of the graph and the value shown are misleading. This aims to fix it. I hate PBI.

# 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.

# Other

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

Adds first day month for reporting purposes

Related work items: #28181

models/reporting/general/schema.yml

@@ -641,6 +641,13 @@ models:
         data_tests:
           - not_null
 
+      - name: first_day_month
+        data_type: date
+        description: |
+          First day of the month corresponding to the date field.
+        data_tests:
+          - not_null
+
       - name: client_type
         data_type: string
         description: |
@@ -1479,6 +1486,13 @@ models:
         data_tests:
           - not_null
 
+      - name: first_day_month
+        data_type: date
+        description: |
+          First day of the month corresponding to the date field.
+        data_tests:
+          - not_null
+
       - name: id_deal
         data_type: character varying
         description: Id of the deal associated to the host.