ETag Caching Support

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.

No implementation files, tests, or client entry points are identified in the issue. First locate the client request and retry paths, then determine the cache scope and persistence behavior before defining tests that verify unchanged resources are not re-fetched and changed resources still update.

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