Add a sending domain

Registers a domain for sending and returns it with the DNS records required to verify ownership and configure DKIM. Responds with 201. Supplying a connection attaches the domain to that DNS account; the domain must sit inside the zone given with it. When Samva cannot register another sending domain right now, this responds with 429 and no domain is created. A banned organization is refused with 403. namespaceId places the domain in a namespace, and only that namespace's keys and organization-wide callers reach it; a namespace-scoped key always adds to its own namespace.