(4/5) [nexus] Consider Affinity/Anti-Affinity Groups during instance placement (#7446)

Pulled out of #7076

Modifies the instance placement logic to consider affinity and
anti-affinity groups.
This is still technically "internal-only", because the HTTP endpoints to
create affinity groups are, as of this PR, still unimplemented.

Author: smklein

nexus/db-queries/Cargo.toml

@@ -25,6 +25,7 @@ futures.workspace = true
 internal-dns-resolver.workspace = true
 internal-dns-types.workspace = true
 ipnetwork.workspace = true
+itertools.workspace = true
 macaddr.workspace = true
 oxnet.workspace = true
 paste.workspace = true