RSS: Refactor how we construct the blueprint we hand off to Nexus (#9399)

This is mostly a refactor of how RSS generates the blueprint it hands
off to Nexus. At the point we need to construct a blueprint, we have a
`ServicePlan` and a `SledPlan` (both renamed imports from `Plan` structs
in other modules). We have to combine these to construct the blueprint:
`ServicePlan` contains all the sled configs but not the sled IDs, and
`SledPlan` contains all the sled IDs. Both contained maps keyed by each
sled's underlay address, so constructing a blueprint was a two step
process:

* Call `build_sled_configs_by_id(sled_plan, service_plan)` to match up
the two maps via sled underlay addresses, producing a single map
containing both sled IDs and sled configs
* Pass that combined map into a helper function that actually
constructed the blueprint

However, this is largely unnecessary: `ServicePlan` did originally know
all the sled IDs too - it just threw that information away. This PR
changes `ServicePlan` to keep track of the sled IDs also, removing the
need to join it against `SledPlan` later. The helper function that
constructs a blueprint is now a `to_blueprint()` method on `ServicePlan`
itself.

(The motivator for this change is that I want to add some additional
fields to `BlueprintSledConfig` that are also available to
`ServicePlan`, and doing so is a lot easier if we already have this
refactor in place.)

There should be no behavioral changes in this PR, except for an extra
small change that snuck in while I was here: Previously, RSS was asking
each sled for its inventory and then separately asking for its role
(i.e., scrimlet vs gimlet). But at some point we added the role to the
inventory, so this PR removes the `get_sled_role` endpoint altogether,
and RSS reads the role from the inventory it just fetched. (I did double
check that the `inventory` and `get_sled_role` handlers read from
exactly the same bit of information to determine the sled's role.)

This is staged on top of #9391 to avoid merge conflicts touching some of
these same bits.

Author: jgallagher

nexus/mgs-updates/src/test_util/host_phase_2_test_state.rs

@@ -127,7 +127,7 @@ impl HostPhase2TestContext {
             .version_policy(dropshot::VersionPolicy::Dynamic(Box::new(
                 dropshot::ClientSpecifiesVersionInHeader::new(
                     omicron_common::api::VERSION_HEADER,
-                    sled_agent_api::VERSION_MULTICAST_SUPPORT,
+                    sled_agent_api::latest_version(),
                 ),
             )))
             .start()