feat: adjustable rule_group interval

[wbollock]

This allows for more expensive SLO recording rules by letting users create a custom Prometheus rule_group.interval, as rule group default or per type of rule.

https://prometheus.io/docs/prometheus/latest/configuration/recording_rules/#rule_group

Now instead of all rule groups using the default global evaluation interval, a custom interval can be set on all three types of recording rules for an SLO.

If no interval is set then the global default will be assumed, matching current Sloth behavior.

Resolves: #367

Edit: I didn't touch the kubernetes operator side and probably need adjustments there

[wbollock]

@slok when you have a chance could you take a look and let me know what you think?

[anosulchik]

looks like an amazing feature, cc @slok