feat: expose kDrive file revision metadata

[OpenCow42]

Summary

• decode optional revised_at and etag fields on KDriveFileItem
• expose optional with resources on last-modified, favorites, my-shared, and shared-with-me listings
• include files.etag in the default advanced-listing resource set
• document ETag as an opaque conditional-upload token and preserve compatibility when resources are omitted

Why

The File Provider needs the same server ETag on detail and listing paths so it can construct authoritative content versions and send conditional If-Match replacements without timestamp heuristics.

Validation

• swift test (558 tests passed)

Stack

• Builds on feat: expose Retry-After response metadata #49 so provider retry handling can consume Retry-After metadata and ETag support together.
• The potassiumProvider hardening PR will pin this branch until both package PRs merge and a versioned package release is available.