Add running custom scripts with a cron

Allow running custom scripts/commands in the workspace at a `cron`. 
This may be used to run scheduled backups in the background.

```hcl
resource "coder_agent_script" "backup" {
    agent_id = coder_agent.dev.id
    name = "backup"
    script = <<EOF
        rclone sync /path/to/source s3:bucket
    EOF
}
```

Related to #9287