PHP: better debug info for Message objects in protobuf C-extension

See https://github.com/protocolbuffers/protobuf/issues/12714 and https://github.com/protocolbuffers/protobuf/pull/12718, which resolved this problem for the native library.

When the protobuf c-extension is enabled for PHP, calling var_dump on a protobuf message object does not output any useful information:

// Calling var_dump on a Protobuf message (extension enabled):
php > $timestamp = new Google\Protobuf\Timestamp();
php > $timestamp->setSeconds(12345);
php > var_dump($timestamp);
object(Google\Protobuf\Timestamp)#1 (0) {
}

Ideally this would output something like this:

object(Google\Protobuf\Timestamp)#12 (1) {
  ["seconds"]=>
  int(12345)
}

Start by reading issue 12714 and pull request 12718, then reproduce the shown var_dump behavior with the PHP C-extension and a populated Google\Protobuf\Timestamp. Done means Message objects expose useful field values in var_dump output, matching the demonstrated seconds example.