Go-Library: Add the ability for basic modifications to a csaf-document-representing go-structure

[JanHoefelmeyer]

Specifically, given a csaf-document via a structure as defined via https://github.com/csaf-poc/csaf_distribution/issues/559, the library is supposed to be able to

• [ ] add elements
• [ ] edit elements or
• [ ] remove elements.
• [ ] It should also be able to sort the keys where applicable.

This will be important for some of the key functions for the library as mentioned in https://github.com/csaf-poc/csaf_distribution/issues/367#issue-1711582988