[IMP] fields, test_new_api: add ondelete support for m2m

Each model of the relation can define `ondelete` for its `column2` foreign key.

This way the behavior is similar to m2o where the target of the `ondelete` is
the `comodel` foreign key.

Some models were already defining `ondelete` 'restrict' on m2m fields and after
review it appears the behavior introduced in this commit is the expected
behavior for them.

The 'restrict' from product.product has to be removed however because the rest
of the code needs adaptation now that it is properly restricted. These changes
will be done on a separate PR.

closes odoo/odoo#35654

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>

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