Merged PR 2676: Propagates Billing Country information

# Description

Propagates Billing Country information in unified_user and user_host intermediate models. This is a necessary step towards providing KPIs segmented by Billing Country.

# 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.

Related work items: #19082

models/intermediate/core/schema.yaml

@@ -2111,6 +2111,21 @@ models:
         description: |
           ID of the country in which the Host is billed. 
           In some cases it's null.
+      - name: billing_country_name
+        data_type: string
+        description: |
+          Name of the country in which the Host is billed.
+          In some cases it's null.
+      - name: billing_country_iso_2
+        data_type: string
+        description: |
+          ISO 3166-1 alpha-2 country code in which the Host is billed.
+          In some cases it's null.
+      - name: billing_country_iso_3
+        data_type: string
+        description: |
+          ISO 3166-1 alpha-3 country code in which the Host is billed.
+          In some cases it's null.     
       - name: account_currency_iso4217
         data_type: string
         description: |