Add in SFTP support

@jibran No you don't. It's all handled through listeners. There is no explicit definition for the field because its derived.

Following a discussion with @nterbogt I'll create a separate module for it because of its dependency on 'key'. We don't want to be bloating data pipelines out and forcing modules on people.

@jibran We haven't added any tests for any other source resources. The test itself would be redundant because the only thing you could mock is getResource.

Closing as now implemented as https://www.drupal.org/project/data_pipelines_sftp →

mortim07 → closed merge request !20