Add sled-agent API for attached external subnets

[bnaecker]

This covers two things:

• Integrating Attached (External) Subnets opte#911, which has the attached subnet API (plus a few other small API changes)
• Adding the sled-agent server APIs for attaching subnets. I think the most reasonable endpoints would be to PUT / DELETE the entire set of subnets attached to an instance, and then a PUT or POST along with a DELETE for a single subnet attached an instance. The former would back the Nexus background task described in Nexus background task for managing external subnets in OPTE #9582, and the latter the public Nexus API for attaching / detaching subnets one at a time, described in Nexus external API for external subnet CRUD #9579