Adding support for Sklearn linear regression in EvaDB

[jineetd]

Starting this PR for integrating sklearn linear regression for EvaDb.

[jineetd]

Currently the integration test is failing with the following error:

TypeError: Singleton array array('rental_price', dtype='<U12') cannot be considered a valid collection.
============================================================================================= short test summary info ==============================================================================================
FAILED test/integration_tests/long/test_model_train.py::ModelTrainTests::test_sklearn_regression - evadb.executor.executor_utils.ExecutorError: Singleton array array('rental_price', dtype='<U12') cannot be considered a valid collection.

Seems like something is wrong with return types somewhere. Need to debug further.

[jineetd]

Unit test is passing with these changes.

[xzdandy]

Overall looks great. Add some comments. We are good to merge then. Thanks for the contribution!