Add an AlterViewModel and AlterMaterializedViewModel as available operations

[zschumacher]

It would be amazing that when I change a query on a PostgresViewModel or PostgresMaterializedViewModel, if psqlextra would generate an alter view migration for me. Currently, I have to either do a SeperateDatabaseAndState to alter the view, or manually add a delete and create view operation to a migration.

[zschumacher]

Realizing that Postgres doesn't have an alter view mechanism for modifying the query, but auto dropping and creating the view would be nice!

[Photonios]

This is supported already by the schema editor:

For views:

django-postgres-extra/psqlextra/backend/schema.py

Line 101 in 9b809d1

def replace_view_model(self, model: Model) -> None:

For materialized views:

django-postgres-extra/psqlextra/backend/schema.py

Line 120 in 9b809d1

def replace_materialized_view_model(self, model: Model) -> None:

I didn't wrap them in migration operations yet because I haven't figured out how to cleanly detect the query changing. You're free to open a PR to at least at the operations for manual use :)