feat: Use IMAP APPEND command to upload sync messages (#5845)

Why:
- With IMAP APPEND we can upload messages directly to the DeltaChat folder (for non-chatmail
  accounts).
- We can set the `\Seen` flag immediately so that if the user has other MUA, it doesn't alert about
  a new message if it's just a sync message (there were several such reports on the support
  forum). Though this also isn't useful for chatmail.
- We don't need SMTP envelope and overall remove some overhead on processing sync messages.

src/sql/migrations.rs

@@ -991,6 +991,20 @@ CREATE INDEX msgs_status_updates_index2 ON msgs_status_updates (uid);
         .await?;
     }
 
+    inc_and_check(&mut migration_version, 119)?;
+    if dbversion < migration_version {
+        sql.execute_migration(
+            "CREATE TABLE imap_send (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                mime TEXT NOT NULL, -- Message content
+                msg_id INTEGER NOT NULL, -- ID of the message in the `msgs` table
+                attempts INTEGER NOT NULL DEFAULT 0 -- Number of failed attempts to send the message
+            )",
+            migration_version,
+        )
+        .await?;
+    }
+
     let new_version = sql
         .get_raw_config_int(VERSION_CFG)
         .await?