Build a virtual file plugin

Build a virtual file plugin that can be used to serve files that don't exist on disk. The plugin needs to support directory listings (file discovery) and file reads. This would be invaluable for testing agents with different instructions, skills, etc off of a declarative Dev Proxy configuration which can be easily swapped without complex and error-prone filesystem operations.

On Windows, we could potentially implement this using ProjFS.

This is likely OS-specific work so I suggest we keep it separate from Dev Proxy. Let's create a separate repo under dev-proxy-tools.