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!