* refactor: store per-file column set in catalog
Together with the table-wide schema and the partition-wide sort key, this should
be everything we need to read a parquet file directly into memory
without peeking any file-level metadata.
The querier will use this to directly load parquet files into the read
buffer.
**WARNING: This requires a catalog wipe!**
Ref #4124.
* refactor: use proper `ColumnSet` type
Marco Neumann