Added

• Added a nrfutil config package-index list subcommand which prints out the current package index configurations. It uses YAML as a display format but that is not guaranteed to be stable so for consumption by other programs it is preferrable to use the --json flag to get JSON output.
• Added a nrfutil config modifier list subcommand which prints out the current modifier configurations. It uses YAML as a display format but that is not guaranteed to be stable so for consumption by other programs it is preferable to use the --json flag to get JSON output.