[FEATURE REQUEST] Support OAS tags structure

**Is your feature request related to a problem? Please describe.**
Tags definitions are not compatible with OAS tags. When you are building tools to parse both specifications this is a problem

**Can't it be tackled using specification extensions?**
Nope

**Describe the solution you'd like**
Either apply the same structure (i.e. define top level tags and then use them as a simple array of tags names in the messages) or remove them.

No files, tests, or entry points are identified in the issue. Compare the current top-level tag definitions with the message tag arrays, then review the OAS tag structure and decide whether AsyncAPI should adopt it or remove the definitions; done means the specification has one clear, compatible tag model.

Written by the indexing model from the issue text.