Add drop_missing_schema feature to capture_sample_data.

[prescod]

Similar to other bulk data tasks, this one can now deal with fields or objects specified in an extract definition which are not in the org.

Unknown sobjects warn rather than crashing noisily because it would have taken a lot of code refactoring to crash noisily. One reason is that our internal schema hides objects that have no data.

[salesforce-cla[bot]]

Thanks for the contribution! It looks like @prescod is an internal user so signing the CLA is not required. However, we need to confirm this.