server/bigquery: support JSON column type

### Is your proposal related to a problem?

[BigQuery native JSON support](https://cloud.google.com/bigquery/docs/reference/standard-sql/json-data) is in preview. However GraphQL Engine metadata can't track BigQuery tables that have a JSON column. I guess the current version hasn't supported this type yet.

```
{
"code": "unexpected",
"error": "cannot continue due to new inconsistent metadata",
"internal": [
{
"definition": "analytics",
"name": "source analytics",
"reason": "Inconsistent object: unexpected exception while connecting to database: GetMetaDecodeProblem \"Error in $.schema.fields[6].type: invalid type \\\"JSON\\\"\"",
"type": "source"
}
],
"path": "$.args[0].args"
}
```

### Describe the solution you'd like

N/A

### Describe alternatives you've considered

Use String type instead. BigQuery supports JSON conversion [functions](https://cloud.google.com/bigquery/docs/reference/standard-sql/json_functions#parse_json) to convert back and fore String and JSON.

### If the feature is approved, would you be willing to submit a PR?

No