Fix: Postgres restore command to support custom usernames (#7438)

[Kntntn]

Changes

• Updated StandalonePostgresql restore commands to explicitly connect to the postgres database using -d postgres.
• Updated the corresponding unit test to verify the correct command generation.
• This prevents FATAL: database "user" does not exist errors when importing data with a custom username that does not have a matching database name.

Issues

• fix [Bug]: Postgres backup import fails when using custom database name and username #7438