Go-Library: Add the ability to import a csaf-document into a go-structure and export it

[JanHoefelmeyer]

This issue depends on #559.

Any csaf-document should be able to be read into that go-structure, and any go-structure should be able to be saved or exported as csaf-document.

This will allow modification and creation of csaf-documents via the csaf-go-library, see #367 (comment)

Note that the standard-name when saving the document as file must confirm to the pattern defined in https://docs.oasis-open.org/csaf/csaf/v2.0/os/csaf-v2.0-os.html#51-filename

Also note that when saving, the keys of the document need to be able to be sorted where applicable.