Data Sharing: Add actor URIs when sharing content URIs

[oneiros]

See #36 (comment)

When receiving content URIs, discovery FASP must make sure that the original author agreed to have this context indexed. It might be beneficial to have the fediverse server send the matching actor URIs alongside the content URIs, so FASP can perform this check before trying to fetch the content.

I will try to keep this in mind and come back to it when starting the implementation of the first discovery FASP.