[SQLite] Expose online backup API

https://www.sqlite.org/c3ref/backup_finish.html#sqlite3backupinit

```rust
impl SqliteConnection {
  async fn backup(&mut self, target: &mut Self) -> crate::Result<()>;
}
```

 * Do we need more than this simple API? The C API is quite involved and I wonder how much really needs to be exposed.