⭐️ allow customisation of scan interval

[chris-rock]

fixes #653

You can now pass in alternative values to cnspec serve to configure the timer and its splay.

> cnspec serve --help
Start cnspec in background mode.

Usage:
  cnspec serve [flags]

Flags:
  -h, --help                    help for serve
      --inventory-file string   Set the path to the inventory file
      --splay int               randomize the timer by up to this many minutes (default 60)
      --timer int               scan interval in minutes (default 60)

Global Flags:
      --api-proxy string   Set proxy for communications with Mondoo API
      --auto-update        Enable automatic provider installation and update (default true)
      --config string      Set config file path (default $HOME/.config/mondoo/mondoo.yml)
      --log-level string   Set log level: error, warn, info, debug, trace (default "info")
  -v, --verbose            Enable verbose output

To run server from CLI:

> cnspec serve --timer 30 --splay 30
→ start cnspec background service
→ scan interval is 30 minute(s) with a splay of 30 minutes(s)

When cnspec is used as a service, it is easier to configure the timer and the splay in the configuration:

api_endpoint: https://us.api.mondoo.com
scan_interval:
  timer: 5
  splay: 10
auto_update: true
...

[arlimus]

so cool to have this configurable now