Add external link option for IBN

[ndricimrr]

Description

Changes proposed in this pull request:

• Adds possibility to define external links for intent navigation

So for intentMapping config:

 {
      semanticObject: 'External',
      action: 'view',
      externalLink: { url: 'https://www.sap.com', openInNewTab: true }
}

The following should navigate to sap.com in a new tab:

LuigiClient.linkManager().navigateToIntent('External-view')
LuigiClient.linkManager().navigate('/#?intent=External-view')
Luigi.navigation().navigate('/#?intent=External-view')

Related issue(s)

Fixes #2936

[JohannesDoberer]

LGTM!