CLOUDNS: populate zone cache when creating zone (#3331)

Hi @pragmaton!
While reviewing all the `ZoneCreator` implementations, I noticed that
the CLOUDNS provider has an incomplete caching implementation for zones.
The provider is populating the cache once on first access. Any zones
that are created will not be readable in the same life-cycle of
dnscontrol. This PR is populating the zone cache after creating a zone.
Would you mind giving this a try and let me know how it goes? Thanks!

Part of https://github.com/StackExchange/dnscontrol/issues/3007

providers/cloudns/api.go

@@ -185,6 +185,11 @@ func (c *cloudnsProvider) createDomain(domain string) error {
 	if _, err := c.get("/dns/register.json", params); err != nil {
 		return fmt.Errorf("failed create domain (ClouDNS): %w", err)
 	}
+	c.Lock()
+	defer c.Unlock()
+	if c.domainIndex != nil {
+		c.domainIndex[domain] = domain
+	}
 	return nil
 }