[C++][Parquet] Process parquet rowgroups without Arrow conversion

### Describe the usage question you have. Please include as many useful details as possible.

I'd like to read a Parquet file and append an Arrow table to the new Parquet file created based on the old file and the new table added as a new row group.
Can I read the Parquet rowgroup by rowgroup, decide to drop any or use them and assemble a new Parquet file without doing the (de)serialization to Arrow?

### Component(s)

C++, Parquet, Python

The issue names no files or tests. Start by tracing the C++ Parquet row-group read/write path and its Arrow conversion boundary, then compare that with the requested append-and-drop workflow. Done means establishing whether the workflow is supported and documenting a supported approach, or producing a clearly scoped change request.

Written by the indexing model from the issue text.