gather_processing_job needs to load existing processing.json

[dbirman]

Describe the bug
On the release-v1.0.0 branch the new gather_processing_job assumes implicitly that no processing.json exists in the output folder. Helen flagged that this is a problem if a user uploads a processing.json file with their data asset.

Expected behavior
The job should load existing processing.json, if it exists, and use the + operator to merge the Processing objects. If there's no existing file it can proceed using the current logic.

[mekhlakapoor]

The airflow dag can check DataProcess object instead of a processing.json so that it can be merged with an existing one if needed