Create unpacker protocol + ext4 unpacker

[adityaramani]

Creates a new Unpacker protocol that defines a single method

func unpack(_ image: Image, for platform: Platform, at path: URL, progress: ProgressHandler?) async throws -> Mount

This change also removes the unpack(...) method from the Image type.

Before

let mount = try await image.unpack(for: platform, at: path)

After

let unpacker = EXT4Unpacker(blockSizeInBytes: 2.gib())
let mount = try await unpacker.unpack(image, for: platform, at: path)

[dcantah]

I think this also returns true if the path is a directory, do we still want to error?

[adityaramani]

I changed the wording to say that --unpack-path should be a new directory and updated the error to match

[dcantah]

Looks great :)