Add capability for serving files

[joshrwolf]

While serving through OCI is nice for a lot of use cases, another common use case is a simple file server.

Add functionality to hauler for an embedded file server (use stdlib if possible) that is capable of serving a filesystem at a user defined root.

To reduce complexity (and maintainability), we shouldn't build a file system interface on top of oci layouts, rather, provide easy to use methods for users to extract File contents from the Store, and then allow them to serve the file system.

This depends on:

• Allow directories to be passed to Files #48 (for ease of use, nobody wants to define a bunch of files)
• expand hauler store copy to operate against multiple "backends" #91