Adds EntryRespository#findByIds()

[nadinengland]

This PR adds findByIds to EntryRepository. 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());

Notable functionality:

• Entries are returned in order of their id's position in the original input argument.
• Missing IDs are omitted from resultant collection.
• Entries are first loaded from Blink, if cached, and remainder are loaded from the DB.

Relevancy to core

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. As I was adding this to the package, parity with the core Repository contract felt good as well: PR on statamic/cms.

[ryanmitchell]

Thanks for the PR.

The goal of this driver is to match the functionality and contract methods provided by core. So for this to merge the core PR would be to be accepted - il leave that for the core team to decide.

[duncanmcclean]

Marking this as a draft until the Core PR has been merged.