Is your feature request related to a problem? Please describe.
As a security tester i would like to configure the start time pattern for scheduledScans more precisely. For now it is possible to configure the time interval (hours) between to scans but the starting point relates to the installation time of the scheduledScan.

Describe the solution you'd like
To be able to configure a more complex schedule for my security scan it would be great to support a crontab syntax as an alternative option.

Example

apiVersion: "execution.securecodebox.io/v1"
kind: ScheduledScan
metadata:
  name: "nmap-scanme.nmap.org-daily"
spec:
  crontab: "0 12 * * *"
  scanSpec:
    scanType: "nmap"
    parameters:
        # Use nmaps service detection feature
        - "-sV"
        - scanme.nmap.org
  historyLimit: 3

Describe alternatives you've considered
none

Additional context

• About Cron: https://en.wikipedia.org/wiki/Cron
• GO Library for parsing https://godoc.org/github.com/robfig/cron
• https://docs.securecodebox.io/docs/api/crds/scheduled-scan