Update connnect-bigquery.md

add info that base64 string is supported for bq json upload. 

Resolves  #5640

website/docs/docs/cloud/connect-data-platform/connnect-bigquery.md

@@ -11,7 +11,10 @@ sidebar_label: "Connect BigQuery"
 
 :::info Uploading a service account JSON keyfile
 
-While the fields in a BigQuery connection can be specified manually, we recommend uploading a service account <Term id="json" /> keyfile to quickly and accurately configure a connection to BigQuery.
+While the fields in a BigQuery connection can be specified manually, we recommend uploading a service account <Term id="json" /> keyfile to quickly and accurately configure a connection to BigQuery. You can provide the JSON keyfile in one of two formats:
+
+- JSON keyfile upload: Upload the keyfile directly in its normal JSON format.
+- Base64-encoded string: Provide the keyfile as a base64-encoded string. When you provide a base64-encoded string, dbt decodes it automatically and populates the necessary fields.
 
 :::