Move IndicesStatsRequest to expose CommonStatsFlags directly

Currently IndicesStatsRequest has a method per possible index metric. Internally theses are stored using a `CommonStatsFlags` object. This means that every time we add a stat we need to remember to add a method and remember to map it in `RestIndicesStatsAction` rather we should just expose the underlying flag, just like `NodesStatsRequest.indices()` allowing for some automation in the mapping.
