[#20] Client: Optimize memory usage

Avoid memory allocation, use cache and static

Signed-off-by: Pavel Gross <p.gross@yando.com>

src/FrostFS.SDK.ClientV2/Services/SessionServiceProvider.cs

@@ -21,13 +21,13 @@ internal class SessionServiceProvider : ContextAccessor
         {
             Body = new CreateRequest.Types.Body
             {
-                OwnerId = Context.Owner.ToGrpcMessage(),
+                OwnerId = Context.Owner.ToMessage(),
                 Expiration = args.Expiration
             }
         };
 
         request.AddMetaHeader(args.XHeaders);
-        request.Sign(Context.Key);
+        request.Sign(Context.Key.ECDsaKey);
 
         return await CreateSession(request, args.Context!);
     }