Added `_shards` header to all write responses.

The header indicates to how many shard copies (primary and replicas shards) a write was supposed to go to, to how many shard copies to write succeeded and potentially captures shard failures if writing into a replica shard fails.

For async writes it also includes the number of shards a write is still pending.
can you give an example of the output here? I wonder if we have duplicate information between the shard info and the failures bellow..  also, I'm not sure it should be a top level item here, but rather inline with each op.