Describe the feature: A project selection control as part of the Unified Search system, visible in Dashboards, Discover, Lens, Maps

Describe a specific use case for the feature:

When I have configured cross project search
And I am viewing analytics (dashboard, etc)
I need run-time control over the scope of the analytics
So I can not have to call my admin or duplicate dashboards to see specific projects' scope of data

More details

Part of #234387

The component in the MVP is going to be mostly a switch. Designs

The users will be able to select if they want to search in all projects (and they will see a list) or origin only.

The component is a unified search component and should have a flag to be enabled or not (as there might be applications that do not want to render it but also it should be visible only in serverless)

The endpoint to get the list of projects are already merged in serverless https://github.com/elastic/elasticsearch-serverless/pull/4521 (_project/tags) and it is behind the serverless.cross_project.enabled feature flag.

This issue is tracking only the component and not the distribution of the filter in the various apps. We will track this in another issue

This UI will eventually create project routing filters with a syntax like:

  "project_routing": "_alias:my-project-1234",