Extract json string field in Google Data Studio

I'm trying to analyze my Firestore data in Google Data Studio. I have successfully imported my data from BigQuery to Google Data Studio, but I'm not unable to use it.

Here is my JSON format in GDS :

{
    "document_name": "projects/path",
    "document_id": "01ASsoYefmYy4h8mGTPWzt5d5Xw1",
    "timestamp": "1970-01-01 00:00:00 UTC",
    "event_id": "",
    "operation": "IMPORT",
    "data": "{\"mail\":\"email@gmail.com\",\"creationDate\":1602011942852,\"initialized\":true,\"languageCode\":\"en\"}"
}

I'm trying to use the creationDate or the mail as field but I'm unable to create a field with this because this is a stringified JSON. I tried to find a solution with formula, without success.

How to exploit field in stringified JSON in Google Data Studio ?

Thanks