[FIX] web: kanban: add a onPagerUpdate callback

Use case:
in the reconciliation widget, we need to know when the pager
is updated before the next render because we need to select the
appropriate id to use.

cf the following PR: https://github.com/odoo/enterprise/pull/33631



opw-3031131

closes odoo/odoo#105738

Related: odoo/enterprise#33631
Signed-off-by: Georis François (fge) <fge@odoo.com>