Use natural primary keys from the source data

[vrajmohan]

Currently, we always add a surrogate key to the source data and have that as the identifier for the resources in the API. However, there are cases where the source data will already have a natural primary key (e.g. state abbreviation for a data set on states). In such cases it would be better to use that as the identifier.

This can be implemented as a command line option to apify.