Make ParquetFileReader extensible

[kenwenzel]

Describe the enhancement requested

I wanted to add a cache for ParquetFileReader.getColumnIndexStore(int blockIndex) and I've noticed that I can't access the required blocks field from a subclass.
To accomplish this I had to fallback to reflection as can be seen here:
https://github.com/linkedfactory/linkedfactory-pod/blob/c67b155e3fbf76082e2b24fd412efae5838badc6/bundles/io.github.linkedfactory.core/src/main/java/io/github/linkedfactory/core/kvin/parquet/KvinParquet.java#L881

I would propose to improve the extensibility of ParquetFileReader by changing certain core members (like blocks) to protected or by exposing them through protected getters.

Component(s)

No response

[wgtmac]

It doesn't make a big difference to change private members to protected or public as we enforce backward compatibility for all public APIs. BTW, is it possible to introduce similar approach like #1174