Engine.cs and SecretNumber.cs refactor fix

The fix is made, because some of the methods are not testable with the
previous implementation.

SecretNumber.cs
1) Size property renamed to Length
2) Generate() method renamed to GenerateRandomValue(int length). The
method no longer awaits the user to input the length of the number.
Instead the length is passed as argument.
3) SetLength() method is removed. This was the method that was setting
the secretNumber length duo to user input in the console.

Engine.cs
1) private int ReadSecretNumberLength() method added.
2) in the StartNewGame() method the this.secretNumber,Generate() is
replaces with
int secretNumberLength = this.ReadSecretNumberLength()
this.secretNumber,GenerateRandomValue(secretNumberLength)