[Feature Request] Autogenerate migrations based on current `spanner_orm.Models`

The current model of migration creation being a static file requiring developers to setup the exact changes desired in this change is less than ideal.

A more desirable model for ease of use is to have the package be able to introspect the models (several ways to do this) and generating the delta required to bring the system up to current state from the previous schema revision.

Inspiration: https://github.com/miguelgrinberg/Flask-Migrate & the whole alembic system