Add support for adding generated/identity columns to existing tables

[kvch]

Right now pgroll only supports adding generated columns to new tables. When you add a new generated column, PostgreSQL has to update all rows in the table. This can be resource intensive and may lead to downtime.

We must add a proper migration to support this case. We can either improve add_column operation or introduce a new operation.

[andrew-farries]

#637 contains removed test cases that could be useful for when we implement this.