Add toolbar to PageLayout component

[pfferrari]

What I did

Added toolbar configuration to the PageLayout component to provide a simplified and guided way to manage the top right buttons and dropdown of the apps.
The new toolbar prop added to the PageLayout component is an object with optional buttons and dropdownItems keys to respectively define:

• The set of main level actions, rendered as Buttons.
• The set of nested actions groups, rendered as DropdownItems inside a company generated Dropdown.
  A default responsive behavior is active by default to rearrage the external Buttons inside the dropdown menu in case of mobile viewport.

Example usage

<PageLayout
  ...
  toolbar={{
    buttons: [
      {
        label: 'Add new',
        icon: 'plus',
        size: 'small'
      },
      {
        label: 'Secondary',
        icon: 'pulse',
        variant: 'secondary',
        size: 'small'
      }
    ],
    dropdownItems: [
      [
        {
          label: 'Edit',
          onClick: () => {
            console.log('Edit')
          }
        },
        {
          label: 'Set metadata',
          onClick: () => {
            console.log('Set metadata')
          }
        }
      ],
      [
        {
          label: 'Delete',
          onClick: () => {
            console.log('Delete')
          }
        }
      ]
    ]
  }}
/>

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• Make sure to add/update documentation regarding your changes.
• You are NOT deprecating/removing a feature.

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[gciotola]

👏 well done!