Statsforecast: allow to create model without group column

[ea-rus]

Description

Updated statsforecast handler to accept dataframes without grouping column. Grouping column is added inside of handler.

Usage:

Fix #6948

Because NFLX.xls dataset doesn't contain group values need to create model without 'group by' clause:

CREATE MODEL 
netflix_stock 
FROM files (select * from nflx)
 PREDICT High 
ORDER BY Date 
 HORIZON 30 
USING ENGINE = 'statsforecast';

Usage of model:

SELECT *
FROM mindsdb.netflix_stock as m
JOIN files.nflx as t
WHERE t.date > LATEST

Type of change

(Please delete options that are not relevant)

• 🐛 Bug fix (non-breaking change which fixes an issue)

What is the solution?

(Describe at a high level how the feature was implemented)

Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation, or created issues to update them.
• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• I have shared a short loom video or screenshots demonstrating any new functionality.