[#142] sdk/netmap: Add type aliases for netmap policy

Later on there should be version independent structures.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/netmap/policy.go

@@ -0,0 +1,13 @@
+package netmap
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/v2/netmap"
+)
+
+// fixme: make types instead of aliases to v2 structures
+type PlacementPolicy = netmap.PlacementPolicy
+type Selector = netmap.Selector
+type Filter = netmap.Filter
+type Replica = netmap.Replica
+type Clause = netmap.Clause
+type Operation = netmap.Operation