Allow developers to extend the Site Health screen with their own custom navigation tabs and pages.
This implements a new filter, `site_health_navigation_tabs`, which takes an associated array of tab identifiers/slugs, and tab labels, allowing developers to add their own subpage to the Site Health interface as new tabs.
To output a custom page, or add to an existing page, the `site_health_tab_content` action is triggered whenever the `tab` query argument is present and not empty. This action includes the current tab as its argument, which a developer would match against to only output content when relevant.
Props ramiy for initial patch.
Fixes#47225.
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