ETag Caching Support

[JustinGrote]

We should support ETag caching in the client to not re-fetch origin requests if they have not changed. The question is where this cache should live:

1. Simple in-memory cache (easy to do in PS with a dictionary) to prevent multiple query retries
2. Disk-based (persistent between sessions, do we really need this though? Usually if someone starts a new session, the info is already stale)

pwsh.gallery will implement some long-lived caching for resources like specific catalogItem entries and leaf pages that should never change after first request.