Refactor project structure for a simplier setup and align with avro-kotlin

[jangalinski]

Top level should hold:

• _build dir containing report-aggregators and parent(s)
• all pblished artifcats that do not have a sub namespace should be to level in repo, so no "in between" modules to cluster serializer, plugin, ...
• bom should also be top level
• Use _itest and _examples to mark things that do end-2-end testing or provide additional examples. These names are standard profile names in the maven-parent and can be used to include/exclude features.
• lib should be reserved for wrapping external dependencies when it is otherwise too hard to include/exclude dependencies