Changes to List all service groups

added the new optional header request parameter SHIPPO-ACCOUNT-ID

for the header request parameter SHIPPO-API-VERSION, default value 2018-02-08 was added

the items/allOf[subschema #2]/service_levels/items response's property type changed from no type to object for status 200

added the optional property items/allOf[subschema #2]/service_levels/items/account_object_id to the response with the 200 status

added the optional property items/allOf[subschema #2]/service_levels/items/service_level_token to the response with the 200 status

removed ServiceLevel subschema #2 from the items/allOf[subschema #2]/service_levels/items response property allOf list for the response status 200