[C++][Parquet] Improve LevelDecoder

[AntoinePrv]

Describe the enhancement requested

Lack of basic functionality (Skip, Count) on the LevelDecoder create additional complexity in downstream classes (ColumnReaderImplBase, TypedColumnReaderImpl, TypedRecordReader).

This complexity is prohibitive to other improvements, such as plugging a flat optional definition level bitmap decoder.

Component(s)

C++, Parquet

[cschanhniem]

The LevelDecoder improvement for Parquet should help with reading performance. For repos with wide contribution surfaces, AGENTOWNERS provides a policy framework to route agent-contributed changes to the correct domain experts and files.