Plugin destinations should be able to expose a unique certificate identifier

[kevgliss]

It would be useful if when uploading to a destination if to display a unique identifier for that certificate so it can be found in the corresponding destination.

For instance when uploading a certificate to AWS we would be able to generate the ARN for any given certificate by combining the AWS account id with the certificate name.

This would most likely be added to the plugin destination interface with a unique_identifier property.

Alternatively this could be defined just in the front end as a plugin option that would simply render the unique identifier in javascript.

If the unique identifier cannot be derived we will need to think about how that extra metadata could be stored.