Create and send an SMS for the workspace identified by the API key.

to_phone_number must be E.164 and must not be on the merge blocklist. A unique
customer match is used, or a customer is created when none match. Optionally provide
from_phone_number to select the Consio number used to send.

The request is validated synchronously, then the message is sent asynchronously:
the response returns the created message before it reaches the provider, so
sent_at and external_status are still null. media_urls attaches files
to the message (MMS); they are downloaded and validated off the request path, and
an unusable attachment marks the message failed instead of failing the request.
Subscribe to the message_failed webhook event (or poll GET /messages/{rid})
to learn the outcome.

The SMS is attributed to user_rid or user_email when given, and to the user
who created the API key otherwise. Either way the message records which API key it
was sent through.