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`.