Before this change, we were not fetching service resolvers (and therefore
service defaults) configuration entries for services on members of sameness
groups.
This commit adds a sameness-group config entry to the API and structs packages. It includes some validation logic and a new memdb index that tracks the default sameness-group for each partition. Sameness groups will simplify the effort of managing failovers / intentions / exports for peers and partitions.
Note that this change purely to introduce the configuration entry and does not include the full functionality of sameness-groups.
