Given a site ID or site object, `get_site()` retrieves site data in the same vein as `get_post()` or `get_comment()`. This will allow for clean retrieval of sites from a primed cache when `WP_Site_Query` is implemented.
Adds a `WP_Site::to_array()` method to support multiple return types within `get_site()`.
Props spacedmonkey.
See #35791.
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* A `WP_Site` object initially matches a row from `wp_blogs`.
* A site can be retrieved by its ID through `WP_Site::get_instance()`.
* Adds `sites` to the global cache group and captures instance lookups.
* The multisite bootstrap now ensures `$current_blog` is an instance of `WP_Site`.
Props johnjamesjacoby, jeremyfelt.
See #32450.
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