feat: switch to alembic migrations

[smotornyuk]

@amercader , I mentioned this change at the dev meeting

To make this extension compatible with CKAN after merging sqlalchemy-2.0 PR, at least we have to pass engine to all create and exists calls

But, if ckanext-harvest targets only CKAN >= v2.9, we can switch to alembic migrations. When I had done it, I decided to move all table definitions to the top level of the module.

Finally, I created a second migration. It adds PostgreSQL level ON DELETE CASCADE to all the foreign keys. It's not required for sqlalchemy upgrade, but it really simplifies cleaning the tables manually

[pdelboca]

@smotornyuk I like this one.

I have tested it locally agains the SQLAlchemy 2.0 branch and it is working. I executed a ckan_harvester against a public instance and I found no issues.