feat: adjustable rule_group interval

[wbollock]

This allows for more expensive SLO recording rules by letting users create a custom Prometheus rule_group.interval.

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 sets of recording rules for an SLO.

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

Resolves: slok#367

[wbh1]

Overall, looks good! Since Sloth produces 3 rules groups, I think it'd be nice to support specifying separate intervals for each of them, though. (sloth-slo-meta-recordings, sloth-slo-alerts, and sloth-slo-sli-recordings are the 3 prefixes). For example, you may want an interval of 2m on the SLI recording rules but still 1m on the alerts.

[wbollock]

@wbh1 commits added for per rule group intervals:
a94fee9
ca78450
dc3994b

[wbh1]

Approving as it looks good as-is, but left some comments on form.

[wbh1]

I think you can simplify this with a switch statement like:

    group := ruleGroupYAMLv2{
					Name:             fmt.Sprintf("sloth-slo-sli-recordings-%s", slo.SLO.ID),
					Rules:             slo.Rules.SLIErrorRecRules,
				}
	switch {
	case AlertRulesInterval.String() != "0s":
		ruleGroupIntervalDuration, err = time.ParseDuration(AlertRulesInterval.String())
        if err != nil {
            return fmt.Errorf("could not parse rule_group interval duration for alerts %w", err)
        } else {
            group.RuleGroupInterval = ruleGroupIntervalDuration
        }
	case RuleGroupInterval.String() != "0s":
		ruleGroupIntervalDuration, err = time.ParseDuration(RuleGroupInterval.String())
        if err != nil {
            return fmt.Errorf("could not parse default ("all") rule_group interval duration %w", err)
        } else {
            group.RuleGroupInterval = ruleGroupIntervalDuration
        }
	}

    ruleGroups.Groups = append(ruleGroups.Groups, group)