Convert all XML elements into JSON

[cgi1]

After having a code basis for flat (#3) and recursive (#9) conversion, we now have the basis to basically convert all elements of the XML tree into JSON.

Elements (Pre #6)

• Leaf Elements of the document root (DocumentTitle DONE, DocumentType -> category (Validate CSAF 2.0 6.1.26 Document Category special case #13), DocumentDistribution OPEN, AggregateSeverity (Implement the Aggregate Severity section #39))
• DocumentPublisher (Implement rudimental prototyp for Document Publisher #3)
• DocumentTracking (Implement converter for first recursive XML section: DocumentTracking #9)
• DocumentNotes (Implement the DocumentNotes section #35)
• DocumentReferences (Implement the DocumentReferences section #28)
• Acknowledgments (Implement the Acknowledgments section #29)
• ProductTree (Implement the ProductTree group of sections #26)
• Vulnerability (Brainstorm vulnerability section #30)

[cgi1]

After we agreed during the kickoff (#7) on which input format is final for our PoC implementation, @sustefil will take care splitting the xml elements into smaller implementations. Realization will then be done in branches and merged into the main repo. Please take care about the side-requirements described in #4 when you go into the first hierachie level.

[cgi1]

This issue becomes an EPIC, where @sustefil will split up the different XML elements (1st hierachie) into smaller issues.

[sustefil]

Created a check-box list for the Sections. Each entry should have its corresponding issue, as it's now.

Vulnerability section is bigger, will be probably split into smaller issues when we are on it

[cgi1]

Last one in the last stage: Vulnerability section - Then closing this EPIC 🥳

[zpevma]

Vulnerability element implemented in #56

[cgi1]

DONE HERE 🥳 🎉

Thank you @zpevma and @sustefil for doing the overwhelming amount of implementation work! 🙏
Also, the reviews by @sthagen and @tschmidtb51 greatly enhanced the code quality and made us write better code at all. 👨‍💻