Add a FUSE backend for Stage()

Context

An efficient way of exposing the local CAS cache content, on platforms supporting it, is to use a custom. FUSE layer on top of the cache storage. For better robustness of the buildbox-casd daemon process, that FUSE layer should be handled in a sub-process: buildbox-fuse.

Task Description

• Propose an implementation of StageTree() relying on buildbox-fuse.

Acceptance Criteria

TBD