httpapi: External service api supports pagination
Created by: ryanslade
This just adds support for including AfterID and Limit in api requests for external services.
The callers themselves have not been updated as it's not an issue yet.
There appear to only be two internal callers: internalClient.ExternalServicesList which isn't used anywhere. internalClient.ExternalServiceConfigs which is only used from out GraphQL layer and actually only ends up extracting the code host URLs. If that code path becomes an issue we can write a custom SQL query.