[#24] Client: Implement pool part1

first iteration - base classes and methods

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

src/FrostFS.SDK.ClientV2/Tools/RequestSigner.cs

@@ -76,6 +76,11 @@ public static class RequestSigner
 
     public static byte[] SignData(this ECDsa key, byte[] data)
     {
+        if (key is null)
+        {
+            throw new ArgumentNullException(nameof(key));
+        }
+
         var hash = new byte[65];
         hash[0] = 0x04;