Summary:
Currently, we don't allow setting `pg_enum` in
`ysql_catalog_preload_additional_table_list`.

This diff allows users to preload `pg_enum`.
Jira: DB-18467

Test Plan:
```
./yb_build.sh release --cxx-test pg_libpq-test --gtest-filter "*PgEnum*"
```

Reviewers: myang, sanketh

Reviewed By: myang

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D47124