[REF] point_of_sale: Add partners domain filter

This commit adds a new function to the module.

Currently, there is no way for managers or administrators to customize the list of customers per PoS UI.

This function can be used as a hook and can be inherited to add a domain to filter partners while loading a PoS session.

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closes odoo/odoo#121731

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>