Alerting: Make silence authz service check rule group permissions

[yuri-tceretian]

What is this feature?
This PR updates silence authorization service to tighten up the security and check if user can read rules for which it tries to read or write a silence.
To check that permission, the service has to fetch all rules in the group and check the user's permissions.

Why do we need this feature?
Silences are protected by folder UID scope only. Rules are protected by intersection of folder UID and Data source UID. Therefore, in GE there is a possibility that a user can see and manage silences for rules that it cannot see due to lack of data source permissions.

Who is this feature for?
Grafana Enterprise users

Special notes for your reviewer:
The pattern I chose is similar to what we do elsewhere (e.g. folders): I gather the list of rule UIDs and then resolve them to rule groups. Performance-wise it's ok because we have an index that meets the filter requirement. However, the weak spot here is that with a big number of rule-specific silences, this could exceed limits on SQL queries.

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.