Fetch contacts with expanded URNs from API endpoint

[rowanseymour]

See nyaruka/rapidpro#4511

So then we can tell users if a contact is unsendable (no URN with a channel), likely unsendable (out of the messaging window on default channel) etc

[github-actions]

Without activity, this issue will be closed in 14 days.

[rowanseymour]

Ok here's some activity bot