[neutron] upgrading port binding:profile.allocation value for port-resource-request-groups API extension

Hi Neutrinos! We found a technical challenge during implementing the port-resource-request-groups API extension[1]. That extension changes the format of the port.resoruce_request as well as the format of the port.binding:profile.allocation. The former is a calculated field on the port so that is easy. However the bindig:profile is persisted in the database so data migration is needed. What is the canonical way to do such DB data translation in Neutron? Can we translate the data in place during alembic migration? Or should we do some kind of online data migration when the data is translated by neutron when it is read from the db? cheers, gibi [1] https://review.opendev.org/c/openstack/neutron/+/805637/5