[firestore-bigquery-export] change datatype in changelog table from STRING to JSON

### What feature would you like to see?
Currently the `data`-field in the `xxx_raw_changelog` is of type STRING, but afaik this field always contains JSON data. Since BQ natively supports JSON datatypes it would be great to ship this plugin with a standard datatype JSON.

The `old_data`-field is also of type STRING, so I guess this should also be shipped by default as JSON.

Did some tests with this new datatype on the actual changelog table and it works seamlessly

### Why?
Because it saves us a lot of money (don't have to query the full blob of data) and we want to make use of all native `JSON_XX` functions.