Constructs an ImmutableCredentials object with supplied accessKey, secretKey as a SecureString
            and an optional session token.
            
 Declaration Syntax
 Declaration Syntax| C# | 
public ImmutableCredentials( string accessKey, SecureString secretKey, string token )
 Parameters
 Parameters- accessKey (String)
- secretKey (SecureString)
- token (String)
- Optional. Can be set to null or empty for non-session credentials.