Summary:
`pgschema_name` should be deleted from the `common::Schema` object.
The idea: always load the PG Schema Name from the PG Catalog (available in the `master::SysCatalog`).
`Schema::pgschema_name` should be deleted because it's a data duplication in fact.

This PART-2 includes updated `CatalogManager` handlers:
* `CatalogManager::ListTables`
* `CatalogManager::GetTableInfoFromNamespaceNameAndTableName`
* `CatalogManager::GetTableSchemaInternal`

Test Plan:
Jenkins

./yb_build.sh --cxx-test pgwrapper_pg_tablet_split-test --gtest_filter PgTabletSplitTest.SplitDuringLongScan

Reviewers: mihnea, hsunder

Reviewed By: hsunder

Subscribers: fizaa, yql

Differential Revision: https://phorge.dev.yugabyte.com/D43590