Add artifact cache testing with buildbarn Asset API implementation

Qinustyrequested to merge
qinusty/bb-asset-hub-ci into master

Description

Adds a test case for using a buildbarn cache deployment with the new asset API artifact cache.

Changes proposed in this merge request:

  • Introduce arguments to conftest.py to ensure index/storage artifact caches can be declared independently.
  • Add a test case for buildbarn as an artifact cache

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