[#28] Clients: Make immutable parameters

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

src/FrostFS.SDK.Client/Parameters/ISessionToken.cs

@@ -8,5 +8,5 @@ public interface ISessionToken
     /// member. The session has a limited validity period, and applies to a strictly defined set of operations.
     /// </summary>
     /// <value>Instance of the session obtained from the server</value> 
-    FrostFsSessionToken? SessionToken { get; set; }
+    FrostFsSessionToken? SessionToken { get; }
 }