feat: tenant settings ruler

[alsoba13]

NOTE: base branch of this set to #3946. do not merge

Tenant settings ruler is a thread-safe ruler that obtains rules from tenant-settings service and caches the result for later calls.

It holds a cache for each tenant, that expires after the default time (60s).
Every first call to a tenant for RecordingRules(tenant string) will initialize the per-tenant cache. When calling again the method, the cached rules will be served unless they expired. In that case, cache will be refreshed.

I discarded doing generics here becase (1) I would like to validate the idea first and (2) there's no need to generalize the cache.

Ruler needs a tenant settings client in place, I will create dependencies after @bryanhuhta.

Some detected todos:

• Expiry time by config (default 60s)