[Parquet][C++] Cleanup and reorganization for Parquet encryption code

### Describe the enhancement requested

Based on the comment here:
https://github.com/apache/arrow/pull/44990#issuecomment-2656104178

We could do some improvements to the Parquet encryption code. Quoting from the message:

> I think at some point a large cleanup and reorganization phase would be deserved on the Parquet encryption code:

- [ ] header and source file naming
- [ ] object and method naming
- [ ] reorganizing responsabilities across objects
- [ ] wacky implementation details (e.g. why exactly do we need the mutable `CryptoContext::start_decrypt_with_dictionary_page`?)

### Component(s)

C++, Parquet