NEW PROVIDER: MSDNS (#1005)

* New provider
* Add support for SRV records
* Modify ACTIVEDIRECTORY_PS provider to warn that it is deprecated.

providers/msdns/domains.go

@@ -0,0 +1,8 @@
+package msdns
+
+import "github.com/StackExchange/dnscontrol/v3/models"
+
+func (c *msdnsProvider) GetNameservers(string) ([]*models.Nameserver, error) {
+	// TODO: If using AD for publicly hosted zones, probably pull these from config.
+	return nil, nil
+}