rgw/admin: add num_shards to bucket struct field and format code

This change add a new bucket struct field, so we can use rgw.admin.API.GetBucketInfo()
to get a bucket's num_shards.

Format bucket.go.

Signed-off-by: oneto1 <644739768@qq.com>

rgw/admin/bucket.go

@@ -10,6 +10,7 @@ import (
 // Bucket describes an object store bucket
 type Bucket struct {
 	Bucket            string  `json:"bucket" url:"bucket"`
+	NumShards         *uint64 `json:"num_shards"`
 	Zonegroup         string  `json:"zonegroup"`
 	PlacementRule     string  `json:"placement_rule"`
 	ExplicitPlacement struct {