Update file extensions for sheet, share, member, and virtual file
metadata to use new custom extensions (.st, .sre, .json, .vf) instead of
.bcfg. Also update client-side file extensions for latest info and local
sheets.

- If config file not exist, now return Error

- Convert manual async functions to async fn syntax
- Replace assert_eq!(true/false, ...) with assert!(...) and assert!(!...)
- Fix useless vec warning by using array directly
- All tests continue to pass after optimizations