Add a Trigger for BeforeUnsubscribe on live queries

[sadortun]

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

Feature / Enhancement Description

Add a livrQuery trigger similar to BEFORE Subscribe, to get an event when a client unsubscribe from a LiveQuery.

Example Use Case

We need to fetch external data while we have users connected to LiveQuery, and stop fetching data when the client unsubscribe.

Usage example :
https://github.com/GoPlan-Finance/GoPlan-app/blob/master/backend/src/cloud/Assets/liveQueries/AssetPrice.ts#L217

Alternatives / Workarounds

None ;)

3rd Party References

[mtrezza]

Thanks for suggesting, a useful addition!

Can you also open a PR for the docs?

[sadortun]

Done.

parse-community/docs#833