[5.x] Adds EntryRepository#findByIds()

[nadinengland]

This PR adds findByIds to the EntryRepository contract and Stache implementation. See docs PR.

$items = \Statamic\Facades\Entry::findByIds([ 3, 1, 0, 2 ]);

$this->assertInstanceOf(\Statamic\Entries\EntryCollection::class, $items);
$this->assertCount(3, $items);
$this->assertEveryItemIsInstanceOf(\Statamic\Contacts\Entries\Entry::class);
$this->assertEquals([ 3, 1, 2 ], $items->map->id());

Entries are returned in order of their id's position in the original input argument. Missing IDs are omitted from resultant collection.

Relavancy to other packages

This is part of a wider piece of work I've start to add a fairly large reduction in DB queries to the statamic/eloquent-driver. The principal idea is to route as many components as possible through the repositories. In doing so we can optimise to load directly from the Blink cache, and only defer to the QueryBuilder when we’re missing items.

As I was adding this to the Eloquent Driver package, parity with the core Repository contract felt good as well.

Future considerations

A future goal would be to see the Entries fieldtype use this method and manage the published statuses itself. This could mean we have more occasion to use the blink cache in the eloquent driver solution and also benefit from eager loading see nadinengland/statamic-eloquent-driver#1.

However, this would certainly too much of a breaking change for 5.x, if even desired at all by your team. In the meantime I can just override the behaviour in my projects, but it may be helpful to highlight that approach here for context:

class Entries extends \Statamic\Fieldtypes\Entries
{
    public function augment($values)
    {
        if (config('statamic.system.always_augment_to_query', false)) {
            return parent::augment($values);
        }

        $items = Entry::findByIds($values);

        if ($this->config('max_items') === 1) {
            return $items->first();
        }

        return $items
            ->where(fn ($entry) => $entry->status() === 'published')
            ->values();
    }
}