Use Remote Asset API for artifact and source caches

Jürg Billeterrequested to merge
juerg/remote-asset into master

This is the first step towards the Remote Asset API.

This MR depends on !1976 (merged), !1977 (merged) and buildbox-casd changes.

Fixes #1272 (closed) and #1273 (closed).

Edited by Jürg Billeter

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