Rename configuration key for `transform`

[chinyeungli]

The current configuration for transform are:

column_renamings:
column_filters:
required_columns:

However, since the transform now works with JSON file (not only CSV), perhaps we should rename these keys as the word column doesn't make sense for JSON file.

[srthkdb]

@chinyeungli As discussed, I have renamed column_renamings, required_columns, and column_filters to field_renamings, required_fields, and field_filters respectively. Please see the PR #419

[chinyeungli]

This has been fixed.