Simplify alert route routing

[soniaAguilarPeiron]

Following the UX work done in https://github.com/grafana/gops-ux-team/issues/118, we want to provide a simplified way to link an alert rule directly to a contact point without without having to care about labels, matchers and policies.

As a first iteration, we want to offer users the option to directly select a contact point for sending alert rule instances.
This new option will be only available for Grafana-managed alerts.

Tasks

Beta Give feedback

1. Create feature flag for simplified contact point selection. #77929
   area/alerting internal kata:simplify-alert-rule-routing +3
   
2. Create new tab to select contact point on the alert rule form. #77930
   area/alerting area/frontend internal kata:simplify-alert-rule-routing +4
   
3. Update notification policy tree view according these new auto-generated policies. #77931
   internal kata:simplify-alert-rule-routing +2
   
4. Alerting: Update UI to let user manage order of the rules in a group #51494
   area/alerting area/frontend kata:multi-tenant-ruler +3
   
5. Be able to visualize usage of this new simplified routing #79792
   area/alerting area/backend kata:simplify-alert-rule-routing +3
6. Create API endpoint from which UI can fetch available receivers (contact points) #81405
   area/alerting area/backend internal kata:simplify-alert-rule-routing +4
7. Update UI to fetch available contact points via dedicated API #80188
   area/alerting area/frontend kata:simplify-alert-rule-routing +3
8. Update UI to fetch mute-timings (time intervals) via dedicated API #80189
   area/alerting area/frontend kata:simplify-alert-rule-routing +3
9. Enable simplified routing for all cloud users #84728
   kata:simplify-alert-rule-routing +1

Backend Tasks

Beta Give feedback

1. Alerting: Time intervals API (read only endpoints) #81672
   area/alerting area/backend area/frontend no-changelog +4
   
2. Alerting: Receivers API (read only endpoints) #81751
   area/alerting area/backend area/frontend no-changelog +4
3. Alerting: Support for simplified notification settings in rule API #81011
   add to changelog add to what's new area/backend area/backend/db/migration area/frontend no-backport +6
   
4. Update Alert Rule API to accept notification settings
5. Implement generation of routes based on notification settings
6. Make sure that user cannot interfere into auto-generate routes via API
7. Update provisioning API to support notification settings
8. Update file-provisioning to support notification settings in rules
9. Alerting: Add support for notification settings for alert rules terraform-provider-grafana#1407
   area/alerting +1
   
10. Optimize routes generator to ignore settings with default route options #82803
    area/alerting internal +2
11. Automatically add mandatory labels to group_by in rule's notification settings #84073
    internal +1