Ability to run shell command instead/in addition to rcon commands.

### Enhancement Type

Improve an existing feature

### Describe the enhancement

Hello,
That would be really nice if we could have something like `RCON_CMDS_STARTUP` for shell so that we could for example add custom scripts in a volume and run them when some events are triggered.
I took a look to `rcon-cmds-daemon.sh` and I believe it could be easily adaptable for shell commands since it seems it just call the command prepended by `rcon-cli`.
Either we could add a secondary `run_command` function or just remove the prepend but it woudl kind of break compatibility so I guess first option is better.
Please let me know what you think about it and either way thanks for this wonderful image.

Start by reading rcon-cmds-daemon.sh and the existing RCON_CMDS_STARTUP handling. Determine how event-triggered commands are invoked and how shell commands should coexist with the current rcon-cli behavior. Done means the requested shell-command configuration can run custom scripts without breaking existing RCON commands.

Written by the indexing model from the issue text.