Support for Custom className in FormContainer

### Duplicates

- [X] I have searched the existing issues

### Latest version

- [X] I have tested the latest version

### Summary 💡

While the FormContainer currently provides a form element, I believe it would be beneficial to allow for custom styling by supporting a custom className prop. 

### Examples 🌈

``` tsx
<FormContainer className="my-custom-class">
  ...
</FormContainer>
```
