[CORE] Breaking C++ code to their own Maven modules

For example:

1. Velox backend's Java code can be moved from `cpp/velox` to `backends-velox/src/main/cpp` or so.
2. the C++ RSS writer implementations can be moved from `cpp/velox/shuffle` to `backends-velox/src-iceberg/main/cpp` or so.
3. etc.

This will require huge amount of refactors so could be a indefinite goal. The first reparation work of doing this could be extracting out a core Gluten C++ library that can be imported by other Gluten modules, with headers exported and APIs defined.