[bst-1] cascache.py: allow using Remote Asset for storing refs

Abderrahim Kitounirequested to merge
abderrahim/remote-asset-bst1 into bst-1

Description

This allows using any server implementing CAS and Remote Asset as artifact cache, while keeping compatibility .

This can for example use buildbox-casd from BuildGrid/buildbox/buildbox-casd!205 (merged) as artifact cache.

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