This implements NixOS/rfcs#136. This issue is agreed-upon by the https://github.com/orgs/NixOS/teams/nix-team

• merge nix store add-file and nix store add-path into one nix store add and expose the underlying store options
  • expose as --key value or --value1|--value2
    • make PR to CLI guidelines to encode the convention
  • replace --recursive with --mode nar|flat|drv|git (denoting the output hash mode)
    • hash types following Stabilise nix hash and subcommands #8876
    • make --mode nar the default due to history and to match path-to-string coercion behavior in the language
    • flat is for consuming a single file
    • drv (later) for implementing RFC 92; hashing mode to support adding .drv files
    • git (later) for implementing RFC 133
    • TODO: clarify how to expose outputHashMode, outputHashAlgo, etc. on the command line
• refactoring: nicer names for the concepts in addToStore: https://github.com/NixOS/nix/blob/master/src/libstore/store-api.hh#L517