Repeater field should not regenerate UUIDs unless it actually needs to

[Muffinman]

Currently the Repeater field seems to regenerate the repeater element UUIDs every time the state is hydrated:

protected function setUp(): void
{

  ...

  foreach ($state ?? [] as $itemData) {
      if ($simpleField) {
          $itemData = [$simpleField->getName() => $itemData];
      }
  
      // This generates a new UUID here even if the original `$state` array had a valid one that could have been re-used
      if ($uuid = $component->generateUuid()) {
          $items[$uuid] = $itemData;
      } else {
          $items[] = $itemData;
      }
  }

  ...
}

This is kind of annoying if you ever want to rely on that UUID being constant for a specific repeater item. Within $state here the repeater may already have a valid UUID, does it need to generate a new one?

Possibly we could do something like this? Allowing the existing UUID to be reused if desired (and by default leave current behaviour).

  foreach ($state ?? [] as $key => $itemData) {
      if ($simpleField) {
          $itemData = [$simpleField->getName() => $itemData];
      }

      // Pass in the existing key and let `CanGenerateUuids` decide whether it needs regenerating
      if ($uuid = $component->generateUuid($key)) {
          $items[$uuid] = $itemData;
      } else {
          $items[] = $itemData;
      }
  }

trait CanGenerateUuids
{
    protected Closure | bool | null $generateUuidUsing = null;

    protected Closure | bool $alwaysRegenerateUuids = true;

    public function generateUuidUsing(Closure | bool | null $callback): static
    {
        $this->generateUuidUsing = $callback;

        return $this;
    }

    public function alwaysRegenerateUuids(Closure | bool $callback): static
    {
        $this->alwaysRegenerateUuids = $callback;

        return $this;
    }

    protected function shouldAlwaysRegenerateUuids(): bool
    {
        return $this->evaluate($this->alwaysRegenerateUuids);
    }

    public function generateUuid(?string $existingUuid = null): ?string
    {
        if (! $this->shouldAlwaysRegenerateUuids() && $existingUuid) {
            return $existingUuid;
        }

        if ($this->generateUuidUsing) {
            return $this->evaluate($this->generateUuidUsing);
        }

        if ($this->generateUuidUsing === false) {
            return null;
        }

        return (string) Str::uuid();
    }

    public static function fake(): Closure
    {
        return static::configureUsing(
            fn ($component) => $component->generateUuidUsing(false),
        );
    }
}