Add SPA utils to vue_shared

What does this MR do and why?

Add SPA utils to vue_shared

The Automate section used some utilities to build a SPA and avoid having to manually define a lot of the Router/ breadcrumbs/active navigation items logic.

This MR extracts this logic and move it to vue_shared while abstracting away even further to allow for true reusability.

This part 1 out of 3.

1. Extract reusable SPA initiator to vue_shared (You are here)
2. Replace dom_utils with a new vue application on top of nav items, similar to how injectVueAppBreadcrumbs works
3. Update documentation of Duo Agent Platform and extract it to the Frontend wide documentation with the new setup.
4. Use this new utility for the AI catalog as well

References

#557717

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