fix: change provider value from constant to property

[apodgorbunschih]

The implementation is not fulfilling the abstraction definition:
https://github.com/open-feature/php-sdk/blob/4e25be68937dc4ec405366cb6ba726f66fc0e5c8/src/implementation/provider/AbstractProvider.php#L19

protected static string $NAME = 'AbstractProvider';

The implementation is pointing not to a property but to a constant.

protected const NAME = 'SplitProvider';

This is leading to getMetadata method in the implementation to return all the time "AbstractProvider" value

Reproducing the issues:

<?php
abstract class AbstractProvider {
  protected static string $NAME = 'AbstractProvider';
  
  public function getMetadata()
  {
      return static::$NAME;
  }
}

class SplitProvider extends AbstractProvider
{
    protected const NAME = 'SplitProvider';
}

class FlagdProvider extends AbstractProvider
{
    protected const NAME = 'FlagdProvider';
}

$splitProvider = new SplitProvider();
var_dump($splitProvider->getMetadata());


$flagDProvider = new FlagdProvider();
var_dump($flagDProvider->getMetadata());
?>

[apodgorbunschih]

@tcarrio Could you please check this change. There was a mistake in the overriding of the static property from the Abstract Provider. That lead to $NAME being all the time AbstractProvider instead of FlagdProvider