etags for index.json invalidation, test coverage (#34641)

Implement an alternative strategy to do index.json invalidation.

The current approach of pairs of index.json / index.json.hash is
problematic because it leads to races.

The standard solution for cache invalidation is etags, which are
supported by both http and s3 protocols, which allows one to do
conditional fetches.

This PR implements that for the http/https schemes. It should also work
for s3 schemes, but that requires other prs to be merged.

Also it improves unit tests for index.json fetches.

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