Split ATExecValidateConstraint into reusable pieces

Split ATExecValidateConstraint into reusable pieces

With this, we have separate functions to add validation requests to
ALTER TABLE's phase 3 queue for check and foreign key constraints, which
allows reusing them in future commits -- particularly this will allow us
to perform validation of invalid foreign key constraints in partitioned
tables.

We could have let the check constraint code alone since we don't need to
reuse that for anything at this point, but it seems cleaner and more
consistent to do both at the same time.

Author: Amul Sul <sulamul@gmail.com>
Discussion: https://postgr.es/m/CAAJ_b96Bp=-ZwihPPtuaNX=SrZ0U6ZsXD3+fgARO0JuKa8v2jQ@mail.gmail.com