[parquet] Improve encoding mask API (wrap bare i32 in a struct w/ docs)

[alamb]

It is probably my OOP / old age, but I would find this API easier to deal with if it were wrapped in a type that had a documented API

Something like

struct EncodingMask(i32);

impl EncodingMask {
  fn new(val: i32) -> Self { 
   ...
  }
  fn as_i32(&self) -> i32 {
    self.0
  }
} 

// then you can implement From impls to/from i32 as well as a no allocation iterator

I am happy to help hack this out if you like the idea

Originally posted by @alamb in #8587 (comment)

[alamb]

label_issue.py automatically added labels {'parquet'} from #8587

[alamb]

label_issue.py automatically added labels {'performance'} from #8587

[alamb]

label_issue.py automatically added labels {'next-major-release'} from #8587