Unified Backups -- Configuration: Create Model Objects with Simple Fields

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

Goals

• Establish base class structure of Gitlab::Backup::Cli::Configuration module
• Create a base class/module that may be used to define well-structured configuration objects
• The configuration object base should include ActiveModel::Model
• It should establish the ability to define simple scalar attributes termed fields (see design notes in wiki)
• It does not need to implement nested configurations (sections) yet -- that is handled in #501123 (closed)
• It does not need to detect and load the tool configuration file in the system -- that is handled in #501124 (closed)