Implement "test" endpoint

Overview

Building upon the work done in MODINVOICE-131 and MODINVOICE-132, this story adds basic business logic for testing FTP connectivity.

This story includes implementation of the "test" endpoint only.

"test" - simply try to connect to the configured FTP server using the provided credentials and return a response indicating success/failure.

See the wiki for additional details

NOTE: I'm assuming these API implementations will leverage shared code and should be fairly straight forward once the main BL is done. If this isn't the case we can split these out into one or more separate user stories.

NOTE: It may be challenging to write unit tests for this functionality. It might be helpful to use an embedded FTP server such as Apache MINA to help. This is not a requirement, only a suggestion.

Acceptance Criteria

• FTP support has been added to "test" endpoints
• unit tests are updated
• API tests are updated.