Add optional alerts for JetStream

[mfaizanse]

Pre-requisite:

• https://github.com/kyma-project/kyma/issues/15011

Once we have the critical alerts in-place. Discuss if it would be beneficial to add more alerts for JetStream.

Tasks:

• [ ] Alert when the stream message count goes over a certain number and over a certain time period (1k events pending for over 3 hours).
  • This alert is a warning.
• [ ] Alert when the stream storage goes over a certain threshold.
  • This alert is a warning.