[WIP] Use dns services instead of hard-coded IP addresses

[smklein]

FYI: Not ready for review! Just trying to put the "megabranch" up somewhere while I pull out bits and pieces as they make sense.

Pieces to pull out:

• Adding concept of "Service" to Nexus, internal APIs
• Pulling Nexus' "RuntimeConfig" into omicron-common, so runtime parameters can be set after package time.
  • Nexus is obviously impacted, as are tests...
  • ... and so is sled-agent/src/services.rs, which launches services.
• Extend internal-dns-client to include "multi-client" support (mostly helpers). Also The AAAA / SRV types in names.rs.
• Extending internal-dns to support multi-records for DnsKV, rather than just one record
• RSS re-work

[smklein]

Replaced by #1216