Button - add new properties

For the button component, we want to provide two more properties to support in the future the design system tokens:

className? (optional)
variant?: 'primary' | 'secondary' | 'tertiary';

The className property needs to provide the ability:

for the developer to provide his own class
needs to have the className dcx-button hardcoded (please use the utility classNames)

If the variant property is specified needs to provide the ability to choose between primary|secondary|tertiary so the className will become `dcx-button--[variant]

some examples:

the developer doesn't specify any className so the output will be:
```
<button class="dcx-button">
```
the developer specifies his own class but not a variant:
```
<button class="dcx-button daniele">
```

the developer specifies a variant primary

<button class="dcx-button dcx-button--primary">

the developer specifies a variant secondary

<button class="dcx-button dcx-button--secondary">

the developer specifies a variant primary and his own className
```
<button class="dcx-button daniele dcx-button--primary">
```

Please follow these steps to create your branch:

git checkout release/0.8
git pull
git checkout -b 'feature/button-new-props'

Capgemini / dcx-react-library

Button - add new properties #466