Changes to List scheduled, sent, and failed messages

response property data/items/created_at list-of-types was widened by adding types null to media type application/json of response 200

response property data/items/updated_at list-of-types was widened by adding types null to media type application/json of response 200

the data/items/scheduled_for/anyOf[subschema #1] response's property format changed from date-time to no format for status 200

the data/items/sent_at/anyOf[subschema #1] response's property format changed from date-time to no format for status 200

added the optional property data/items/sender to the response with the 200 status

endpoint added