Depends on #70789

What?

Improves the pages/posts list performance by limiting the query to the fields that are needed.

Why?

To improve performance, as we avoid fetching unnecessary fields like the post content.

How?

Leveraging _fields in the posts query, including a minimum set of required fields plus those fields present in the current view.

Testing Instructions

1. Go to the posts list experiment, or to the pages list in the site editor
2. Test that it's working with no errors
3. In the network tab of the browser, verify that the request that queries the posts is smaller in size and faster.

Screenshots or screencast

Before, the posts query fetches the default fields, including the post content, for all posts on the first page of the dataview

After, it will only fetch the required fields, plus an extra request to fetch the whole selected post to preview it:

Note:
Depends on #70738; until it lands, permissions are not properly cached, and there will be one extra OPTIONS request per post in the dataview to check permissions.