Allow adding buttons to the Page header

[lottamus]

In https://github.com/stoplightio/app-gen3/issues/399 we want to be able to use the Page component but its currently missing support for adding buttons.

Suggest updating the Page component to look something like this:

export interface IPage extends IErrorBoundary {
  type: NodeType;
  data: any;

  name?: string;
  srn?: string;
  version?: string;
  versions?: string[];

  headerRight?: React.FunctionComponent

  padding?: string;
  className?: string;
  shadows?: boolean;
  scrollInnerContainer?: boolean;
}

We will also need to support adding these buttons via the Page container too:

elements/src/containers/Page.tsx

Line 10 in 1799cc2

export interface IPage {

[lottamus]

Let's render the buttons above the Page component inside of Explorer rather than passing them into the Page for now