Add a new action type with an associated URL which will be opened as an overlay.

In particular this is useful if further external context should be included with a message.

This action should NOT dismiss the message, therefore it is not a replacement for the dismiss button.

• Add new ActionType with value OPEN_LINK
• Add an optional field link to the ActionDto
• Add handling for action OPEN_LINK OnLaunch-iOS-Client#49
• Add handling for action OPEN_LINK OnLaunch-Android-Client#16
• Add handling for action OPEN_LINK OnLaunch-Flutter-Client#14