Support downloading multiple files

[YDX-2147483647]

When downloading multiple files, Send concatenates all files into a raw stream. This PR checks the files manifest, and split the stream back to normal files.

Technical details

Research about the sever side

https://github.com/timvisee/ffsend/issues/134#issuecomment-1740348270

Implementation

If we are downloading multiple files (if metadata.mime() == "send-archive"):

1. Allocate a temporary directory
2. Prepare paths: Check each file in the manifest, and ask to overwrite if necessary
3. Download the archive (a raw stream) to temp dir
4. Post Process: Split the archive to output dir (fn split(archive: File, files: Vec<ManifestFile>))

Other changes

• Replace extract: bool with enum InvokeStrategy

• Replace file: bool of prepare_path() with strategy: &InvokeStrategy

  [!NOTE]

  When archive feature is dropped, output_dir (file) should be true. I guess there was a mistake when introducing the archive feature.

  https://github.com/timvisee/ffsend/blob/238f1f54d7bde88c0e525dd0d5c4c6aa251a3634/src/action/download.rs#L123-L134

• Expose manifest and its files from metadata API: https://github.com/timvisee/ffsend-api/pull/82

• SplitError, its From trait, etc.

Additional information

Resolves #134

Please merge https://github.com/timvisee/ffsend-api/pull/82 first if you want to merge this PR.