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

#134 (comment)

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.

  ffsend/src/action/download.rs

  Lines 123 to 134 in 238f1f5

  	// Prepare the download target and output path to use
  	#[cfg(feature = "archive")]
  	let output_dir = !extract;
  	#[cfg(not(feature = "archive"))]
  	let output_dir = false;
  	#[allow(unused_mut)]
  	let mut target = Self::prepare_path(
  	&target,
  	metadata.metadata().name(),
  	&matcher_main,
  	output_dir,
  	);

• Expose manifest and its files from metadata API: Expose manifest and its files from metadata API (to support downloading multiple files) ffsend-api#82

• SplitError, its From trait, etc.

Additional information

Resolves #134

Please merge timvisee/ffsend-api#82 first if you want to merge this PR.