Use LevelHistogram throughout Parquet metadata

[etseidl]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Related to #6129. #6105 added a new LevelHistogram struct to represent histograms recently added to the spec. There remain places where this histogram data is still represented with a Vec<i64>.

Describe the solution you'd like
Replace the remaining bare vectors (in PageIndex and ColumnIndexBuilder with the new struct.

Describe alternatives you've considered
The vectors in ColumnIndexBuilder could be left as is since they are closer to the thrift representation, but I think it's cleaner for all level histograms outside actual thrift structs to share the same interface.

Additional context
See #6105 (comment)

[alamb]

label_issue.py automatically added labels {'parquet'} from #6135