Add command to get an attachment (#1080)

* Add command to get an attachment

* Refactor retrieving of attachments to use StreamDetails

* Refactor AttachmentCommand to GetAttachmentCommand

* Minor improvements to GetAttachmentCommand

* Use JSON serializer to serialize binary data

Serializing the stream is better for memory handling than
loading the whole thing into the file.

* Clean up unneeded class

* Added command to doc

Co-authored-by: cedb <cedb@keylimebox.org>