Add support for renaming labels in PostgreSQL enums

[kvch]

We should add support for a simple way to rename a label in an enum. PostgreSQL does not support renaming labels. PostgreSQL users have to drop the old enum and recreate it with the new label.

Other enum related operations are good enough as raw migrations.

[andrew-farries]

How would this behave with multiple-schema versions?

Is this something that should be done with a raw SQL migration rather than adding a new pgroll operation?

[kvch]

Creating a new enum type, dropping an existing enum type and adding new label to an enum is a simple operation, it does not need its own dedicated pgroll migration. Pgroll is meant to support users in migrations that need two schema version at the same time or to avoid unnecessary locking of tables.

Renaming existing labels in an enum type could be a potential new operation, as it consists of several steps (dropping the old and recreating the enum with new label name), and database clients might want to use both old and new label name during the testing phase.