feat: initial content api endpoints

[notnmeyer]

adds the initial content api endpoints. they're feature flagged by team and so folks will get a 401 (and soon a message about requesting access).

➜ task -- campaigns -h

  Manage campaigns

  USAGE


    loops campaigns [command] [--flags]


  COMMANDS

    create [--flags]  Create a draft campaign
    get <id>          Get a campaign
    list [--flags]    List campaigns

  FLAGS

    --debug           Print API request details before sending
    -h --help         Help for campaigns
    -o --output       Output format (text, json) (text)
    -t --team         Team key name to use

➜ task -- email-messages -h

  Manage email messages

  USAGE


    loops email-messages [command] [--flags]


  COMMANDS

    get <id>               Get an email message
    update <id> [--flags]  Update a draft email message

  FLAGS

    --debug                Print API request details before sending
    -h --help              Help for email-messages
    -o --output            Output format (text, json) (text)
    -t --team              Team key name to use

[nalanj]

Just noting the underlying API design here is not good, but you can't do anything about it.