This PR improves Grafana Alerting integration:
- get alerting contact points "on fly" instead of keeping them in db
- add ability to connect more than one contact point
- add ability to create new contact point on create Grafana Alerting
integration
- show warnings in integration settings for non-active contact points
- remove creation alerting notification policies on create Grafana
Alerting integration
## Checklist
- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)
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Co-authored-by: Rares Mardare <rares.mardare@grafana.com>