Add openAddPanelFlyout to Embeddable API

[streamich]

• Expose .openAddPanelFlyout() method in embeddable plugin start contract.
• Use this new method in Dashboard Angular controller.

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[ppisljar]

Thank you for contributing to this issue, however, we are closing this issue due to inactivity as part of a backlog grooming effort. If you believe this feature/bug should still be considered, please reopen with a comment.