[Integrated Alerting] Alerting migration to Native Grafana Alerting

User story: 

• As a PMM user, I've already added some Alerts Rules to IA. Now, I need them to be in Grafana Alerting so I can edit them using the UI.
• As a PMM user, I have some  "notification channels" in my IA, so I expect them to be converted to Contact points in Alerting. 
• If External Alertmanager was used,  as a user, I expect its configurations be moved to Grafana Alerting settings, so my alerting system is not impacted. 

UI/UX: designer

Acceptance criteria

• Alert rules from IA are available in Grafana Alert tules UI
• In Contact Points - the user has added all his existed Notification channels from IA
• External Alert manager settings  (URL and rules) migrated to Grafana  (Admin section), and no user was impacted by this change. 
•  alertmanager inside PMM - ? 

Out of scope: 

• tbd

Suggested implementation:
TBD

How to test:

1. install PMM version that has IA (for example 2.29)
2. Enable Integrated Alerting
3. Create multiple alert rules (with/without filters, with different labels, etc...)
4. Upgrade pmm up to 2.31 version
5. Run migration script: https://github.com/percona/pmm/pull/1234/files
6. Check that all rules were migrated, and everything works

Details:
TBD