doc(jans-auth-server): show asynchronous calls to third-party in custom scripts

### Description

Custom scripts allow to inject code directly into AS execution. It gives a lot of flexibility but also responsibility.
Recently there were incident when UpdateToken script tried synchronously call third-party to check balane. This check balance call holded execution for 60seconds. It ruins AS perfomance.

If there are calls to third-party it is better to use asynchronous approach instead of synchronous.
1. let access_token to be returned but schedule asynchronous check. If check is OK -> do nothing. If failed -> revoke token.
2. it is bad practice to call third-party for each call. Results must be cashed when possible.

We need documentation and sample script that shows how asynchronous approach works.