Feature: Add ability for Repository simulator to invoke validation on attachment

[stl-steve-moore]

When a Provide and Register transaction is received, the metadata is automatically validated. Attachments are ignored.

Add a property to tell the Repository simulator to call an external service to validate the attachment. What that means will depend on the external service.

Validating just the attachment is the first goal. A secondary goal would be to validate the combination of the attachment and metadata for consistency. I am not suggesting changing the existing metadata validation.