Ruler: add support for caching rule group contents

[56quarters]

Is your feature request related to a problem? Please describe.

Currently, the contents of rule groups for tenants are not cached. This means an API call to return all configuration for rule groups must perform a list call for object storage (this is sometimes cached) and then N additional object storage requests. For tenants with a large number of rule groups, this can result in a huge number of calls.

Most rule group configurations don't change very often. This makes them a good candidate for caching. However when they are changed, that needs to be reflected immediately in API results. So if we want to add caching for rule groups we also need a way to invalidate their cached contents.

Describe the solution you'd like

We should:

• Add functionality to CachingBucket to invalidate cached object storage contents on mutations. (PR)
• Unify "direct" and "cached' ruler storage backends in the Ruler so that cache invalidation works correctly. (PR)
• Add further cache invalidation logic to prevent data races. (#591, #9546, #601, #9575)
• Configure ruler storage to use caching for Get operations (PR)
• [optional] Increase the cache TTL for rule groups cached via Get operations to something longer than a single evaluation interval

Describe alternatives you've considered

Doing nothing is simpler and doesn't require opening up the can of worms that is cache invalidation.