[IMP] intrastat_product: Define the src_dest_country_id field readonly=False

Example use case:
- Our company is based in France
- Create a sales order with a delivery address in France
- Confirm the sales order
- Validate the picking
- Create the invoice
- Although the src_dest_country_id is France, there are specific cases that may require
changing the country (even when the invoice is confirmed); for example, if we have
shipped the goods to a customer's logistics warehouse in France but then the customer
moves the goods to another country (Ireland, for example), in that case we will need
to change the country and define Ireland because it must appear on the Intrastat
declaration.

TT58076

Author: victoralmau

intrastat_product/models/account_move.py

@@ -28,6 +28,7 @@ class AccountMove(models.Model):
         string="Origin/Destination Country",
         compute="_compute_src_dest_country_id",
         store=True,
+        readonly=False,
         help="Destination country for dispatches. Origin country for arrivals.",
     )
     src_dest_region_id = fields.Many2one(