[YSQL] ysql_dump: Database user isn't backed up

[yifanguan]

Jira Link: DB-2977

Description

See the example below:

CREATE USER abcdefg;
SET SESSION AUTHORIZATION abcdefg;

// backup && restore

SET SESSION AUTHORIZATION abcdefg;
ERROR:  role "abcdefg" does not exist

Currently, ysql_dumpall has the functionality to dump global objects: database roles and tablespaces. However, yb_backup.py runs ysql_dump for each database to back up its schema, and it runs ysql_dumpall to only back up tablespaces when --use_tablespaces flag is passed. In source code of yb_backup.py:

self.run_ysql_dumpall(['--tablespaces-only', '--file=' + sql_tbsp_dump_path])