chore: add --audience-filter arg to campaigns create/update

[notnmeyer]

➜ task -- campaigns create -h

  Create a draft campaign

  USAGE


    loops campaigns create [--flags]


  FLAGS

    --audience-filter       Inline JSON string with an ad-hoc audience filter. Pass "null" to clear.
    --audience-filter-file  Path to a JSON file with an ad-hoc audience filter
    --audience-segment-id   Audience segment ID to target. Pass "null" to clear.
    --campaign-group-id     Campaign group ID
    --debug                 Print API request details before sending
    -h --help               Help for create
    --mailing-list-id       Mailing list ID to target. Pass "null" to clear.
    -n --name               Campaign name
    -o --output             Output format (text, json) (text)
    --schedule-at           Send at the given ISO 8601 timestamp (e.g. 2026-07-01T12:00:00Z)
    --schedule-now          Send immediately when published
    -t --team               Team key name to use