Сhange CountDistinct to Count platform function

[peter-gribanov]

Сombine Spec::countDistinct() and Spec::COUNT() into one method/class.

Upgrade

• The Happyr\DoctrineSpecification\Operand\CountDistinct class was removed, use
  Happyr\DoctrineSpecification\Operand\PlatformFunction\Count instead.

• The Spec::countDistinct() method was removed, use Spec::COUNT() instead.

  Before:

  new CountDistinct('field_name');
  Spec::countDistinct('field_name');

  After:

  new Count('field_name', true);
  Spec::COUNT('field_name', true);

• The COUNT function as argument to Spec::fun() is not longer supported, use Spec::COUNT() instead.

• The COUNT function as argument to Happyr\DoctrineSpecification\Operand\PlatformFunction is not longer supported,
  use Spec::COUNT() instead.

  Before:

  new PlatformFunction('COUNT', 'field_name');
  Spec::fun('COUNT', 'field_name');

  After:

  new Count('field_name');
  Spec::COUNT('field_name');