Added support for Mongo pre-image in change stream

Mongo 6.0 now officially supports emitting document pre-image in change streams. 
This will allow Mongo to emit a similar before field as in other DBs.
 
I think a potential implementation can be 

• a config flag to enable capturing pre-image from change stream i.e. change_stream_preimage 
• an extra before field in the change event to captures the pre-image. This behavior is the same as other DBs e.g. postgres, mysql.