* 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>