Add support for s3: URIs in tough/tuftool for targets and metadata resources

[ginglis13]

Description:

Currently, tough and tuftool require http(s) URLs or local file paths for accessing TUF repository targets and metadata. Adding native support for s3:// URIs could extends functionality of the library/tool to support usecases where a TUF repository is only hosted on S3.

Proposal:

Allow s3:// URIs to be used anywhere targets or metadata resources are currently specified, including:

• Repository base URLs in tough client
• Target file locations
• Metadata file locations
• tuftool commands that reference remote resources

Example Usage:

// tough client
let repository = RepositoryLoader::new(
    "s3://my-bucket/tuf-repo",
    // ...
).load().await?;

# tuftool
tuftool download \
    --metadata-url s3://my-bucket/tuf-repo/targets \
    --target-url s3://my-bucket/tuf-repo/targets \
    out-dir

Alternatives considered

• Download the entire repo via s3 sync and use tuftool local file transport support.
  • this isn't feasible for the use-case in mind given the size of large TUF repos
• Use S3 pre-signed URLs
• Keep the status quo