[php-src] Issue #21323: PDO\SQLite: add support for backup API

Issue: https://github.com/php/php-src/issues/21323
Author: bohwaz

### Description

SQLite3 class provides support for the SQLite3 backup API: https://www.php.net/manual/en/sqlite3.backup.php

```
 SQLite3::backup(SQLite3 $destination, string $sourceDatabase = "main", string $destinationDatabase = "main"): bool
```

It would be nice to have the same in PDO\SQLite:

```
PDO\SQLite::backup(PDO\SQLite $destination, string $sourceDatabase = "main", string $destinationDatabase = "main"): bool
```