Implement and document extras for the new query view page

[simonw]

• Plan for getting the new JSON format query views working #2109

[simonw]

I've now shipped the new query JSON without any extras: https://latest.datasette.io/fixtures.json?sql=select+*+from+facetable

There aren't actually that many extras that make sense for the query page. These are the most obvious:

• query - adding a {"sql": "...", "params": ".."} block, as seen on https://latest.datasette.io/fixtures/facetable.json?_extra=query&_size=0
• duration_ms to show the duration - which isn't an extra on table right now but should be.
• metadata - https://latest.datasette.io/fixtures/facetable.json?_extra=metadata&_size=0

[simonw]

Punting this out of the 1.0a3 alpha for the moment.

[simonw]

Need ?_extra=columns too since I'm removing "columns" from the default query JSON in #2136