Support config files for client customization

Currently, updating client docker images or flags requires modifying `client-cmds/*-cmd.sh` files, which creates uncommitted changes in the main branch and requires a new PR for every flag change. We should support optional config files that can override default settings like docker images, flags, etc. These config files would be gitignored and local-only, allowing users to customize their setup without modifying tracked files. If a config file doesn't exist, the system falls back to defaults from `*-cmd.sh` files, so the behavior remains unchanged for users who don't need customization.