Add pending message data to daily and user_daily stats

[heyitsmebev]

Currently, the Recent Activity table displays only the counts for delivered and failed messages. We aim to include the count of pending messages to provide a clearer representation of the daily messages sent.

Backend work:

Please update these endpoints to also have "pending" count
get_service_notification_statistics_by_day
"/service/{0}/statistics/{1}/{2}".format(service_id, start_date, days)
get_user_service_notification_statistics_by_day
/service/{0}/statistics/user/{1}/{2}/{3}".format(service_id, user_id, start_date, days),

Frontend work:

Display pending stats to chart

[heyitsmebev]

@jonathanbobel researching/providing color options for pending that is accessible
@heyitsmebev will coordinate to get "pending" into the endpoint with backend dev and update the chart