Changes to Preview First Messages

removed the optional property messages/items/fallback_reason from the response with the 200 status

added the optional property messages/items/write_error to the response with the 200 status

added the new optional request property sender_name

added the new optional request property booking

the response property messages/items/st_customer_id became optional for the status 200

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

added the optional property messages/items/member_key to the response with the 200 status

endpoint added