fix: Missing credentials for downloading the java-db from private registry

[tom1299]

Description

The main container with the image command of a scan job fails when pulling the java-db from a private registry because of missing credentials. This PR introduces a second init container analogous to the one that downloads the trivy-db but downloads the java-db if it is to be pulled from a private registry.
This results in the main container not having to download the java-db since it is up to date.

Related issues

Checklist

• I've read the guidelines for contributing to this repository.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[tom1299]

Please note that I added an dedicated test case for this case instead of adding an additional on to the main test cases in TestPlugin_GetScanJobSpec because I thought that a test case focusing only on the actual functionality seemed easier to understand. But I can move the test to TestPlugin_GetScanJobSpec if that fits better