[12.x] Allow closures for values in firstOrCreate and createOrFirst

[gcavanunez]

This PR allows firstOrCreate & createOrFirst to accept a closures as the $values parameter, so that lazy evaluation of expensive operations are only happening when a record actually needs to be created.

The use case

When using firstOrCreate with values that involve expensive operations (API calls, encryption) are evaluated eagerly, even when the record already exists and the values are never used.

To work around this today, you have to leave firstOrCreate and split the lookup from the creation:

$location = Location::query()->firstWhere('address', $address);

if ($location) {
    return $location;
}

return Location::create([
    'address' => $address,
    'coordinates' => Geocoder::resolve($address),
]);

With this PR, you can wrap the values in a closure to defer evaluation while preserving the single call:

$location = Location::firstOrCreate(
    ['address' => $address],
    fn () => ['coordinates' => Geocoder::resolve($address)],
);