Return sql and dataframe if call is successful

[danielcorin]

The setup is a bit complicated to describe in full. In summary, I loaded the Chinook dataset into my database, then followed the guide for getting setup with OpenAI and Postgres quite closely.

Before the code change

vn.ask(question="which employee has the most customers?")

None

After

vn.ask(question="which employee has the most customers?")

('SELECT support_rep_id, COUNT(customer_id) AS customer_count\nFROM customer\nGROUP BY support_rep_id\nORDER BY customer_count DESC\nLIMIT 1;',
    support_rep_id  customer_count
 0               3              21,
 None)

[danielcorin]

Same as https://github.com/vanna-ai/vanna/pull/207/files