Putaway strategy method

This module brings back a putaway strategy option that was removed from Odoo's standard Inventory app in version 12. It works on putaway rules, the configuration records that tell Odoo where to store a product when it arrives in the warehouse.

What changes for users:

• On the putaway rule form (the same screen used to set up storage rules), a new Method field appears right after the destination location field.
• The Method field is required and defaults to "Fixed Location", which is the only choice currently offered.
• The destination location field is only shown when the method is "Fixed Location"; if another method were added later, that field would be hidden.

The module extends the stock.putaway.rule model and adds the Method selection field to it. It also adds a view that inherits the standard putaway rule list/form view to place the new field and control the visibility of the destination location.

No new menus, reports, scheduled jobs, emails or automatic computations are added. The module only affects the putaway rule configuration screen in Inventory. Because only one method value exists, the practical effect today is limited to showing and storing the method on each rule, keeping the configuration ready for additional strategies.