Add method to fetch records from a batch

[caioeps]

This is what will be returned from this method:

[{"Id"=>["RECORD_ID_1"], "AField__c"=>["123123"]}, {"Id"=>["RECORD_ID_2"], "AField__c"=>["123123"]}, {"Id"=>["RECORD_ID_3"], "AField__c"=>["123123"]}]

Documentation on it: https://developer.salesforce.com/docs/atlas.en-us.api_asynch.meta/api_asynch/asynch_api_batches_get_request.htm

Note: It'd be nice to add some tests to it later on.