[#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/audit/json.go

@@ -1,26 +1,14 @@
 package audit
 
 import (
+	"github.com/nspcc-dev/neofs-api-go/rpc/message"
 	audit "github.com/nspcc-dev/neofs-api-go/v2/audit/grpc"
-	"google.golang.org/protobuf/encoding/protojson"
 )
 
 func (a *DataAuditResult) MarshalJSON() ([]byte, error) {
-	return protojson.MarshalOptions{
-		EmitUnpopulated: true,
-	}.Marshal(
-		DataAuditResultToGRPCMessage(a),
-	)
+	return message.MarshalJSON(a)
 }
 
 func (a *DataAuditResult) UnmarshalJSON(data []byte) error {
-	msg := new(audit.DataAuditResult)
-
-	if err := protojson.Unmarshal(data, msg); err != nil {
-		return err
-	}
-
-	*a = *DataAuditResultFromGRPCMessage(msg)
-
-	return nil
+	return message.UnmarshalJSON(a, data, new(audit.DataAuditResult))
 }