[#196] pkg/client: Extend Client interface with Conn method

Add `Conn` method to `Client` interface which must return the underlying
connection. Implement new method on the core structure. `Conn` can be used
to control the connection (e.g. for closing).

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/client/raw.go

@@ -1,6 +1,8 @@
 package client
 
 import (
+	"io"
+
 	"github.com/nspcc-dev/neofs-api-go/rpc/client"
 )
 
@@ -12,3 +14,8 @@ func (c *clientImpl) Raw() *client.Client {
 
 	return c.raw
 }
+
+// implements Client.Conn method.
+func (c *clientImpl) Conn() io.Closer {
+	return c.raw.Conn()
+}