[VL] Merge modules `gluten-arrow` and `backends-velox`

This will mean merging Velox's CPP code base `cpp/core` and `cpp/velox` altogether at the same time.

We had some discussions like https://github.com/apache/incubator-gluten/issues/4201 which becomes more reasonable over time. At the very beginning, `cpp/core` and `cpp/velox` were split for extensibility purpose, but we don't seem to have chance to leverage this extensibility anymore. ClickHouse backend is unlikely going to rely on `cpp/core` in the future either. Maintaining this extensible design is costly for the community and has no clear reason.

The work will have two parts:

1. Merge Maven module `gluten-arrow` into `backends-velox`.
2. Merge `cpp/core` to `cpp/velox`.