ruler: support /config/api/v1/rules/{namespace}/{groupName} for local storage

[dimitarvdimitrov]

The endpoint /config/api/v1/rules/{namespace}/{groupName} isn't supported for local rules storage.

mimir/pkg/ruler/rulestore/local/local.go

Lines 97 to 100 in bc241d9

	// GetRuleGroup implements RuleStore
	func (l *Client) GetRuleGroup(_ context.Context, _, _, _ string) (*rulespb.RuleGroupDesc, error) {
	return nil, errors.New("GetRuleGroup unsupported in rule local store")
	}

Since this is a read-only endpoint the ruler should be able to support it. This is how the ruler accesses all the rule groups for a namespace for a user. The logic can be reused

mimir/pkg/ruler/rulestore/local/local.go

Line 117 in bc241d9

func (l *Client) loadAllRulesGroupsForUser(ctx context.Context, userID string) (rulespb.RuleGroupList, error) {