feat(server): allow accounts to opt out of Kura cache

What changed

• Added a :kura_cache_opt_out account-level feature flag helper.
• Updated cache endpoint resolution so opted-out accounts ignore mirrored Kura endpoints and fall back to custom cache endpoints or the default hosted endpoints.
• Added account endpoint tests for both fallback paths.

Why

Accounts are automatically routed to Kura once a ready Kura server mirrors its public URL into account_cache_endpoints. We need an operational escape hatch for accounts that should keep using the existing cache path even if they have a Kura server available.

Impact

Operators can enable :kura_cache_opt_out for a specific account in FunWithFlags. The Kura UI and provisioning flag remains separate, so this only affects which cache endpoint the CLI receives.

Validation

• git diff --check -- server/lib/tuist/accounts.ex server/lib/tuist/feature_flags.ex server/test/tuist/accounts_test.exs
• Started mix test test/tuist/accounts_test.exs, but it was interrupted before completion while preparing the PR.