ARROW-5702: [C++] parquet::arrow::FileReader::GetSchema()

[romainfrancois]

This adds this method to parquet::arrow::FileReader :

/// \brief Return arrow schema for all the columns.
  ::arrow::Status GetSchema(std::shared_ptr<::arrow::Schema>* out);

might be useful for e.g. #4627

[nealrichardson]

Seems uncontroversial on the surface, but shouldn't there be a test added for it?

[wesm]

Agree that it doesn't seem controversial. A simple unit test and fixing the build seems in order

[wesm]

I fixed this and added a usage to a unit test

[wesm]

+1. This change isn't causing the other failures we're seeing

[romainfrancois]

Thanks. Sorry I did not get a chance to get back to it soon enough.