Struct support - vspec2ddsidl

This issues concerns struct support for vspec2ddsidl.

Required work:

- Define/propose how it shall be handled (possibly in this PR)
- Implement and test
- Make sure that expected output format is described in documentation
- If feasible, add at least one test case checking output

_VSS Struct support is expected to be officially released for VSS 4.0, late April. To identify and manage remaining a work number of issues have been created to facilitate more detailed discussion and to identify volunteers for implementing/reviewing/testing._

@SebastianSchildt will check if the ones originally contributing vspec2ddsidl are interested

Start by locating the vspec2ddsidl implementation and its existing tests. Resolve and document how VSS structs should be represented, then implement and test the support, including at least one output-format case. The issue provides no file or test path, so repository exploration and design discussion are required.

Written by the indexing model from the issue text.