imapnotify: Use JSON type for extraConfig (#4238)

Prior to this change, it was impossible to nest attrsets in
accounts.email.accounts.<name>.imapnotify.extraConfig. However,
goimapnotify's configuration is JSON-based, and the recommended
configuration has:
```
  "tlsOptions": {
    "rejectUnauthorized": true
  },
```

This change changes the type from an attrset of str/int/bool to the
JSON type provided by nixpkg's `pkgs.formats.json`.

modules/services/imapnotify-accounts.nix

@@ -1,4 +1,4 @@
-{ lib, ... }:
+{ pkgs, lib, ... }:
 
 with lib;
 
@@ -31,7 +31,7 @@ with lib;
     };
 
     extraConfig = mkOption {
-      type = with types; attrsOf (oneOf [ bool int str ]);
+      type = let jsonFormat = pkgs.formats.json { }; in jsonFormat.type;
       default = { };
       example = { wait = 10; };
       description = "Additional configuration to add for this account.";

modules/services/imapnotify.nix

@@ -99,7 +99,9 @@ in {
     };
 
     accounts.email.accounts = mkOption {
-      type = with types; attrsOf (submodule (import ./imapnotify-accounts.nix));
+      type = with types;
+        attrsOf
+        (submodule (import ./imapnotify-accounts.nix { inherit pkgs lib; }));
     };
   };