[mapistore-python][REST]: Correct argument modification
[mapistore-python][REST]: Add attachment POST operation
[mapistore-python][REST]: Add attachment PUT operation
[mapistore-python][REST]: Add attachment GET operations (get attachment from ID & get attachments in a message)
[mapistore-python][REST]: Add attachment HEAD operations (check if attachment exists & count attachments)
[mapistore-python][REST]: Add attachment DELETE operation
[mapistore-python][REST]: Implement create_attachment + attachment class
[mapistore-python][REST]: Implement attachment save