Use capabilities service in artifact cache

Background

Currently CASRemotes check whether a remote is enable for pushing to, based on the reference status, even for artifact cache services. These should you the new capabilities service once that's merged (!1410 (merged)) instead.

Task description

• Change check_remote to allow use different checks in subclasses
• Use the Capabilities service for checking whether an artifact cache is configured for remote.

Acceptance Criteria

Artifact caches use capabilities service to check whether BuildStream can push to them.

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