Add Str.Pad Overload

```csharp
string Pad(int length, string str = null, string padStr = null, PadTypes type = PadTypes.Right);
```

The older api will Obsolete, removed in 2.0 version.

```csharp
string Pad(string str, int length, string padStr = null, PadTypes type = PadTypes.Right)
```