[Request] Session event emission

Emit different event by the clientSession.

Currently there is only a flag `hasEnded` in the document `$session` indicating if the current session is still ongoing or not.
forcing to check at regular interval for the change of flag if there is need to do action **after** a transaction has been committed or not.

**What is the expected behavior?**

It would be much easier if the `$session` was en eventEmitter, it could emit event type `end|committed|closed|aborted` etc...

**What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.**
`node v 15.5.1, mongoose v 5.9.24`

Start by tracing the Mongoose $session transaction lifecycle and the existing hasEnded flag; the issue names no source file, entry point, or test. Define the event contract for session end, commit, close, and abort, then verify that consumers can reliably observe each outcome after transaction completion.

Written by the indexing model from the issue text.