Save compute output checksums in PodResult

[guzman-raphael]

This issue is to add evaluating checksums for any outputs generated from a compute run and storing them into the model.

Specifically:

// model.rs
enum Artifact {  // renamed from `Input` to be more general so use with outputs is less confusing
    Unary(Blob<FileOrFolder>),
    Collection(Vec<Blob<FileOrFolder>>),
}

struct PodJob {
    // ...
    output_dir: Blob<FolderOnly>, // no checksum, just a rename from previous `output_stream_path` since that is what this actually is
    // ...
}

struct PodResult {
    // ...
    output_dir: Option<Blob<FolderOnly>>, // added with checksum
    output_stream_path: Option<BTreeMap<String, Artifact>>, // added with checksums
    // ...
}

Depends on #14, #54