[MLOPS-320] Optimization of generation grid column schemas process, change logic on front-end and back-end

[pawel-laczkowski]

Back-end

• Introduced a new columns_metadata dictionary in the Experiment model to store user-defined columns information (metrics and parameters).
• Updated the logic for creating and deleting iterations to ensure that columns_metadata is correctly maintained during these operations.
• Added assertions in tests to validate the correctness of the new logic.

Front-end

• Updated the process for generating grid column schemas.
• Modified the projects datastore logic to ensure that columns_metadata is updated when iterations are deleted.