[Guided onboarding] Add a callback for "Continue" button

[yuliacech]

When a user doesn't complete a guide step at once, they can use the "continue" button to get back to the url at the beginning of the step. The solution page logic might need to reset their "step content" flow so the code needs to broadcast this event. We can add a callback or an observable that is called when the user click the "continue" button.

[elasticmachine]

Pinging @elastic/platform-onboarding (Team:Journey/Onboarding)

[alisonelizabeth]

Closing as not planned