RSS should consume "pool of addresses for Nexus"

[smklein]

In a handful of spots, this is being hardcoded as a single address. We will want to support multiple external IPs for Nexus, and should instead consume a pool of IPs.

[rmustacc]

In addition to allowing us to specify a specific IP pool that isn't being used for customers, we should assume that operators will want to explicitly set and control these external IPs and select addresses from the pool.

[smklein]

In addition to allowing us to specify a specific IP pool that isn't being used for customers, we should assume that operators will want to explicitly set and control these external IPs and select addresses from the pool.

This tracks - I figured there would need to be:

• A setup / transfer mechanism to get a minimal set of this info from RSS -> Nexus, for bootstrapping
• A general purpose API within Nexus for operators to use - ideally, similar to the customer-visible IP pool APIs - for managing these "used-for-Oxide-service" IPs.

[bnaecker]

Just clarifying another point from the networking sync today. We will need to add a way to refer to one or more IP Pools when configuring each individual service. As an example, customers may create more than one IP Pool during RSS, on totally separate networks, and want to have Nexus listen on an address from each pool. So the service configuration data needs to include a list of IP Pools.