GET handler for Drive backend

[qrkourier]

Summary

Implement an auto-index for backend mode drive in zrok. Add a handler for GET / HTTP/1.1 that responds with an index.html listing the available files, so that a drive share is also a web page when accessed in a browser.

Motivation

Currently, a drive share requires manual navigation or tooling to see available files. Providing an index.html enables users to easily browse shared files using a standard web browser, enhancing usability for casual and technical users alike.

Benefits

• Improves the discoverability and usability of drive shares.
• Avoids a parallel/redundant zrok web share for HTTP GET of the same contents of a drive share.

[qrkourier]

Note to self - see if we're already using Caddy internals in drive, and consider doing so if not because Caddy has both WebDAV and indexing modules.