Alerting: Create endpoint to report on datasource health and alert rules

[alexweav]

What happened:
Currently, when loading alert rules for a panel or the alert rules list, the UI must scan all datasources by issuing requests to each one in sequence. It checks to see if the DS is available via the test endpoint, supports alerts, and then requests alerts from its ruler API. This means that opening any such page with several datasources can end up issuing hundreds of requests in sequence every time this page is loaded.

What you expected to happen:
Instead, we should have a single endpoint that gives an aggregated view of datasource availability/health/rules. Then, the UI should be able to just issue a single request to get this info.

How to reproduce it (as minimally and precisely as possible):
Add lots of datasources.
Open the alert list panel and watch the requests go.

Anything else we need to know?:

Environment:

• Grafana version: main
• Data source type & version: n/a
• OS Grafana is installed on: n/a
• User OS & Browser: n/a
• Grafana plugins: n/a
• Others: n/a