Revert "mbsync: remove unneeded extraConfig.channel"

This reverts commit 941c4771ca.

To support backwards compatibility, I need to leave this field/option
in the module, even if it will likely be more confusing to do it this way.

modules/programs/mbsync-accounts.nix

@@ -179,6 +179,20 @@ in {
       '';
     };
 
+    extraConfig.channel = mkOption {
+      type = extraConfigType;
+      default = { };
+      example = literalExample ''
+        {
+          MaxMessages = 10000;
+          MaxSize = "1m";
+        };
+      '';
+      description = ''
+        Per channel extra configuration.
+      '';
+    };
+
     extraConfig.local = mkOption {
       type = extraConfigType;
       default = { };