`json_extract` should use double quotes on Snowflake

I'm using the Stripe package and have set project variables for fields I want to extract from `metadata` columns. It works on most values but isn't extracting anything when the key contains a full stop, e.g. `plan.id`.

I have fixed this locally by adding double quotes to the [Snowflake macro](https://github.com/fivetran/dbt_fivetran_utils/blob/cc2329a92fdaee11ea59fd8a7ce69e8b8fe8fe6c/macros/json_extract.sql#L15):
```
{% macro snowflake__json_extract(string, string_path) %}

json_extract_path_text(try_parse_json( {{string}} ), {{ "'\"" ~ string_path ~ "\"'" }} )

{% endmacro %}
```

Happy to make a PR but I'm not sure whether the problem applies to other databases.