masking: Add CLI tools to manage the proxy

Online help:

    Usage: swh storage masking [OPTIONS] COMMAND [ARGS]...

      Configure masking on archived objects

      These tools require read/write access to the masking database. An entry must
      be added to the configuration file as follow:

              storage:
                …

              masking_admin:
                masking_db: "service=swh-masking-admin"

    Options:
      -h, --help  Show this message and exit.

    Commands:
      clear-request   Remove all masking states for the given request
      history         Get the history for a request
      list-requests   List masking requests
      new-request     Create a new request to mask objects
      object-state    Get the masking state for a set of SWHIDs
      status          Get the masking states defined by a request
      update-objects  Update the state of given objects