Added

• Added caching for the toolchain-index. This can be used as a fallback if the toolchain-manager fails to fetch and/or parse the index.
• Added completion for installed toolchains, which is used by the --ncs-version and --toolchain-bundle-id flags in uninstall, launch and env.
• Added completion for toolchains available for installation based on the most recently cached copy of the current toolchain-index.
• Added a flag to override the current toolchain using its bundle-id: --toolchain-bundle-id.
• Added the config subcommand to allow easy configuration of the toolchain-manager.