Add support for peekQuery and previewQuery

[niekdemelker]

As the title states, Add support for the peekQuery and previewQuery functions.

Sometimes you don´t want to load all there is in the details query, and sometimes you want some extra in the peek, like for example a count for amount of relations. Now the Peek and Preview always load the DetailQuery data, which could be ta much or not enough. All the other Query types have there own functions so this should be no exception.

this could be extended in the \Laravel\Nova\PerformsQueries file with the following backward compatibility., and offcourse the controllers should look for this new function instead of the detailQuery.

public static function peekQuery(NovaRequest $request, $query)
{
    return static::detailQuery($request, $query);
}

public static function previewQuery(NovaRequest $request, $query)
{
    return static::detailQuery($request, $query);
}