[sled-agent] Use qorb for switch zone clients

In #7373, we had a system in a state where 1 of the 3 internal DNS servers had no records and 2 of the 3 did. sled-agent early networking code attempts to resolve dendrite in internal DNS to find the switch zone:

https://github.com/oxidecomputer/omicron/blob/21c677e57daffd46f8aed8e465f2c2ee0276bc07/sled-agent/src/bootstrap/early_networking.rs#L264-L271

This appears to have failed due to accepting the response from the 1 of 3 with no data:

20:09:36.420Z WARN SledAgent (ServiceManager): Failed to look up switch zone locations
    error = Error resolving dendrite services in internal DNS: no record found for Query { name: Name("_dendrite._tcp.control-plane.oxide.internal."), query_type: SRV, query_class: IN }
    file = sled-agent/src/bootstrap/early_networking.rs:233
    retry_after = 17.59195738s

qorb handles this more gracefully and would allow us to succeed via the responses from the other two servers; we should replace these uses of Resolver with a qorb pool.

Start in sled-agent/src/bootstrap/early_networking.rs, especially the resolver use around lines 233 and 264-271, and trace how switch zone locations are looked up. Replace the Resolver uses with a qorb pool so lookup succeeds when one internal DNS server returns no records while the others do; verify the early networking lookup behavior.