Add ADR for namespace packages

[atextor]

---

Description

Adds a decision record that describes how valid Aspect Model namespace packages
are structured.

Fixes #305.

Type of change

Please delete options that are not relevant.

• Decision record
• [-] Bug fix (non-breaking change which fixes an issue)
• [-] New feature (non-breaking change which adds functionality)
• [-] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [-] My code follows the style guidelines of this project
• [-] I have performed a self-review of my own code
• [-] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [-] My changes generate no new warnings
• [-] I have added tests that prove my fix is effective or that my feature works

[Yauhenikapl]

LGTM!

[external-Bogdan-Toma]

I see no problem in switching to the new API and removing our in house zip handling. Some zip validation cases from catalog will be handled by sdk and some by catalog (or maybe all by sdk).