Give an example of JSON input for BigQuery source

## Tell us about the problem you're trying to solve
While entering in the JSON for authentication to BigQuery, I am not sure how much of the JSON to paste into the text box. Without instructions, I may only paste the key instead of the entire file.

## Describe the solution you’d like
I would like more directions on what part of the JSON file to paste in. I would like to see, in the documentation or UI, explicitly what needs to be pasted in, ideally with an example in the documentation.

## Describe the alternative you’ve considered or used
Other solutions that require similar JSON inputs use a file upload instead. This could reduce error as the user would not typically edit their file before uploading it.