[RW Separation] Honor allocation awareness attributes

[mch2]

Today all shards are considered equally by AwarenessAllocationDecider. With search only replicas being primary ineligible, this will leave an imbalance of write or search availability across awareness attributes. To fix this we need to apply the alloc decision to both writer eligible and search only shard groups independently, while also taking into account nodes marked specifically for search with the cluster.routing.allocation.search.replica.dedicated.include setting introduced to allocate search replicas to set of nodes.