feat: add pluggable cache provider with Tuist backend

Summary

• Add a CacheProvider abstraction in fabrik-cas wrapping the local CAS, with Tuist as the first remote backend (writes mirror to the remote while a local CAS stays in front).
• Add a new tuist module covering OAuth login/logout (TuistAuth, browser prompt) and the cache transport against api/cache/cas, api/cache/keyvalue, and api/cache/endpoints.
• Extend the workspace manifest with a cache_provider field: local, named ({ name, account?, project? }), or direct tuist config (url, endpoint, account, project, token_env, oauth_client_id); defaults to https://tuist.dev and TUIST_TOKEN, validates env-var names.
• Add fabrik auth login|logout <provider> and thread the resolved provider through build, run, test, exec, cache, and deps/* via dispatch.rs and a new cache_provider resolver in the CLI.

Testing

• Not run (not requested)

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