[ADD] partner_firstname for v8

because it's working as is

> Why to not use install hooks?
> odoo/odoo@4105b5f 
> <https://github.com/odoo/odoo/commit/4105b5f0285821d8056c721b5fd50b9cae902988>
>

How?


Anyway, I think ```_set_default_value_on_column``` is the right place to 
do it, as we are doing just that: setting a default value for the new 
column ```lastname``` and adding the NOT NULL constraint on it