[Search] Introduce search navigation plugin

[TattdCodeMonkey]

Summary

This PR introduces the search_navigation plugin which will be used as a dependency for other search plugins to render the 'classic' page side nav when their UIs are used in Stack. This work is a pre-requisite for the Search team to refactor pages out of the enterprise_search plugin and instead use the search_____ plugins to render pages and routes for both Serverless and Stack.

Additionally this new plugin is nested in the x-pack/search_solution/ folder which is where we ill want to start migrating search specific plugins to for better organization and separation.

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