Add sorting to partition store

Extend the partition store to parse the "sort" query parameter as a
sorting condition. Dot notation is used to denote the object field.
Preceding the key with "-" denotes descending (reverse) order.

Example sorting by name:

GET /v1/secrets?sort=metadata.name

Reverse sorting by name:

GET /v1/secrets?sort=-metadata.name

All values are converted to strings and sorted lexicographically.

pkg/stores/partition/store.go

@@ -128,6 +128,7 @@ func (s *Store) List(apiOp *types.APIRequest, schema *types.APISchema) (types.AP
 	}
 
 	list := listprocessor.FilterList(stream, opts.Filters)
+	list = listprocessor.SortList(list, opts.Sort)
 
 	for _, item := range list {
 		item := item