mbsync: add extraConfig option for easier-to-format options

These are options that can be handled by the `genSection` function in
the `genAccountFunction`, so they are left to the user to decide.
Most of these are made on a global basis anyways.

modules/programs/mbsync-accounts.nix

@@ -89,6 +89,22 @@ let
         '';
       };
 
+      extraConfig = mkOption {
+        type = extraConfigType;
+        default = { };
+        example = ''
+          {
+            Create = "both";
+            CopyArrivalDate = "yes";
+            MaxMessages = 10000;
+            MaxSize = "1m";
+          }
+        '';
+        description = ''
+          Extra configuration lines to add to <emphasis>THIS</emphasis> channel's
+          configuration.
+        '';
+      };
     };
   };