Views argument plugin needs to return ID, not store entity

Problem/Motivation

You get an error when using the store contextual filter argerument using the default value "Store filter".

Error: Object of class Drupal\commerce_store\Entity\Store could not be converted to string in implode() (line 83 of core/modules/views/src/Plugin/views/argument/NumericArgument.php).

Steps to reproduce

1. Add a view of type "Commerce Product"
2. Add a contextual filter of "Product: Stores" and choose "Provide default value" and choose this module's argument, "Store filter"
3. Load the view to get the error

Proposed resolution

Return the store ID, not the entity.

Remaining tasks

Update store filter default argument class to return the store ID.

I shall post a MR in a moment. Thanks for the excellent module by the way! :)