Thanos Rule: posibility to setup rules

[sancyx]

Is your feature request related to a problem? Please describe.
Right now there's only a string property rule.rules to specify rules.

Describe the solution you'd like to see

a) Using serviceOverrides it's possible to add a config map as volume, so would be enough to map rules property to rule-file param: Rules string json:"rules,omitempty" thanos:"--rule-file=%s".

b) Add a new property where user can specify a config map containing rule file(s) which is then mapped as a volume and set --rule-file ccordingly.

c) Support setting up rules from PrometheusRule resources.