Download & decrypt uploaded attachements.

- Parse the "Chat-Upload-Url" header
- Add message::schedule_download function to schedule download job
- In the job download, decrypt, save

src/headerdef.rs

@@ -34,6 +34,7 @@ pub enum HeaderDef {
     ChatContent,
     ChatDuration,
     ChatDispositionNotificationTo,
+    ChatUploadUrl,
     Autocrypt,
     AutocryptSetupMessage,
     SecureJoin,