feat: allow to specify name when creating backup

[ym]

This add a optional flag --name so name can be specified when creating a backup.

E.g.

➜  pscale-cli git:(feat/backup-flag-name) go run cmd/pscale/main.go backup create _ main           
!! WARNING: You are using a self-compiled binary which is not officially supported.
!! To dismiss this warning, set PSCALE_DISABLE_DEV_WARNING=true

Backup 2026.06.03 09:02:30.838 was successfully created.

➜  pscale-cli git:(main) ✗ go run cmd/pscale/main.go backup create _ main --name pkk
!! WARNING: You are using a self-compiled binary which is not officially supported.
!! To dismiss this warning, set PSCALE_DISABLE_DEV_WARNING=true

Backup pkk was successfully created.