Use Secrets for sensitive data inside GoogleBigQuery connection config

[hugobarauna]

When we drag and drop the JSON credentials file to the Google BigQuery connection config, the Smart Cell generates code that contains sensitive data, like the private_key and private_key_id.

Now that we have the Secrets feature, we could use that feature somehow instead of saving sensitive data inside the notebook.

What do you think?

[aleDsz]

I've been thinking about this and storing the JSON itself inside a secret could work (detecting that's a JSON, and decoding after the System.fetch_env!/1). @josevalim WDYT?

[josevalim]

We would have to support secret uploads in the secret modal. Perhaps via a configuration parameter set when the modal is open by Kino. Then the file is uploaded and its contents stored in the env var.

/cc @cristineguadelupe