[#263] v2: Support new rpc library

Implement `message.Message` interface on all structures and use new methods
for conversion instead of functions. make `Unmarshal` and JSON methods to
use encoding functions from `message` library. Remove all per-service
clients and implement `rpc` library of the functions which execute NeoFS API
RPC through new RPC client. Remove no longer used gRPC per-service clients.

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

v2/rpc/session.go

@@ -0,0 +1,28 @@
+package rpc
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/rpc/client"
+	"github.com/nspcc-dev/neofs-api-go/rpc/common"
+	"github.com/nspcc-dev/neofs-api-go/v2/session"
+)
+
+const serviceSession = serviceNamePrefix + "session.SessionService"
+
+const (
+	rpcSessionCreate = "Create"
+)
+
+func CreateSession(
+	cli *client.Client,
+	req *session.CreateRequest,
+	opts ...client.CallOption,
+) (*session.CreateResponse, error) {
+	resp := new(session.CreateResponse)
+
+	err := client.SendUnary(cli, common.CallMethodInfoUnary(serviceSession, rpcSessionCreate), req, resp, opts...)
+	if err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}