Introduce AlertManager v2 integration with improved internal behaviour
it's using grouping from AlertManager, not trying to re-group alerts on
OnCall side.
Existing AlertManager and Grafana Alerting integrations are marked as
Legacy with options to migrate them manually now or be migrated
automatically after DEPRECATION DATE(TBD).
Integration urls and public api responses stay the same both for legacy
and new integrations.
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Co-authored-by: Rares Mardare <rares.mardare@grafana.com>
Co-authored-by: Joey Orlando <joey.orlando@grafana.com>
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