Implement init container support (ADR-0009) - #724

This PR implements ADR-0009 by exposing the initContainers API in the scantypes. It thus closes #707.

Description

ScanTypes now expose another configuration option, initContainers, which allows you to specify a container (or set of containers) that should run before the scan container. Use this to download files onto the scanner or perform other actions in preparation for the scan. It uses the built-in functionality of Kubernetes and does not require any SCB-specific knowledge.

For testing, here's an example application of an nmap scan that scans a set of websites that is downloaded from the web using an initContainer.

apiVersion: "execution.securecodebox.io/v1"
kind: Scan
metadata:
  name: "test-init-containers"
spec:
  volumes:
    - name: test-dir
      emptyDir: {}
  volumeMounts:
    - mountPath: "/test/"
      name: test-dir
  scanType: "nmap"
  parameters:
    - "-iL"
    - "/test/targets.txt"
  initContainers:
    - name: "download-targets"
      image: busybox
      command: 
        - wget
        - "https://gist.githubusercontent.com/malexmave/23246404607929739792f7c5f9750d83/raw/0fac194deeecc5553f96d5ad095a4f8a899dc50f/gistfile1.txt"
        - "-O"
        - /test/targets.txt
      volumeMounts:
        - mountPath: "/test/"
          name: test-dir

I will update the documentation to provide example use cases and create a PR on the relevant repo next.

Checklist

• Test your changes as thoroughly as possible before you commit them. Preferably, automate your test by unit/integration tests.
• Make sure npm test runs for the whole project.
• Make codeclimate checks happy