Unified Backups -- Configuration: Add Nesting To Config Objects (Sections)

Previous Research Issues: #475114 Design reference: https://gitlab.com/groups/gitlab-org/geo-team/-/wikis/Unified-Backups-Configuration-Notes

Goals

• Augment base structure of configuration objects to now allow nested configuration objects (termed sections in the wiki planning notes)
• These may be direct key-to-configuration object references, or they may be arrays of a specific configuration structure
• It does not need to detect and load the tool configuration file in the system -- that is handled in #501124