Add support for bulk downloading of chunks to the client

The metadata server has [an endpoint](https://github.com/cooklang/cooklang-sync/blob/main/server/src/chunks/mod.rs#L138-L158) that allows to download multiple chunks at once using `multipart/mixed` HTML content type (based on https://docs.rs/rocket-multipart/0.2.1/rocket_multipart/ crate).

Need to make client updates, so we support feature fully.

There's a [newsletter issue](https://mailchi.mp/91881dc50992/how-i-developed-a-sync-library-for-cooklang) explaining how it works.