[Fleet] Generalize custom backlink on integration detail page

[flash1293]

In #181195, an onboarding-specific parameter got introduced so the integration detail page can link back to the oblt onboarding flow instead of the list of integrations.

This was done to unblock the work currently in progress, but mid-term features like this should be built in a generalized way.

Tasks:

• Decide on how to represent this feature (passing state via the navigation, a general URL parameter)
• Implement this logic in the integrations plugin
• Adjust the onboarding page to use the new functionality.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)