Origin/fix missing features error
_The documentation is not available anymore as the PR was closed or merged._ please review :) @lhoestq @ola13 thankoo Thanks :) I just updated the test to make sure it works even when there's a column missing, and did a minor change to json.py to add the missing columns for the other kinds of JSON files as well (I moved the code to`self._cast_table`) Thanks Unso! If @lhoestq is happy then I'm also happy :D When I noticed the ping, this PR had already been merged...

Luckily, PyArrow's `read_json` behaves the same when `explicit_schema` is given via `ParseOptions`, so I'm okay with this change (our JSON loader doesn't use `read_json` for decoding JSON in some scenarios, so this manual approach is the right one).

This fixes the problem of when the dataset_load function reads a function with "features" provided but some read batches don't have columns that later show up. For instance, the provided "features" requires columns A,B,C but only columns B,C show. This fixes this by adding the column A with nulls. 