Allow specifying custom encoder when `message_in_json_format=True`

[realsuayip]

This way, the user can provide custom serialization logic for non-serializable extras injected by frameworks. For example Django uses "request" extra in django.request handler which is a type of non serializable WSGIRequest

[xente]

It's not currently planned, but we're open to evaluating it.

In the meantime, if you need this functionality, you can create a custom formatter based on one of the existing ones in the formatters folder and add the extra logic there.

For example, you could create a Django-specific formatter and include it in the formatters folder.