feat: pre-messages / next version of download on demand (#7371)

Closes <https://github.com/chatmail/core/issues/7367>

Co-authored-by: iequidoo <dgreshilov@gmail.com>
Co-authored-by: Hocuri <hocuri@gmx.de>

deltachat-jsonrpc/src/api/types/message.rs

@@ -92,6 +92,9 @@ pub struct MessageObject {
 
     file: Option<String>,
     file_mime: Option<String>,
+
+    /// The size of the file in bytes, if applicable.
+    /// If message is a pre-message, then this is the size of the file to be downloaded.
     file_bytes: u64,
     file_name: Option<String>,