fixes and refactor for api data fetching

[KirilKurkianec]

No description provided.

[KirilKurkianec]

@jontofront you can checkout the branch to confirm that error are gone

[jontofront]

Thanks, erros are gone

[jontofront]

This pull request includes several changes to the custom_components/econet300/api.py file to improve the consistency and clarity of the API data fetching methods. The most significant changes involve replacing the _fetch_reg_key and _fetch_reg_names methods with a new _fetch_api_data_by_key method, and updating various methods to use this new method.

Improvements to API data fetching:

• custom_components/econet300/api.py: Replaced _fetch_reg_key and _fetch_reg_names methods with a new _fetch_api_data_by_key method to standardize the process of fetching API data by key.
• custom_components/econet300/api.py: Updated the get_param_limits, fetch_reg_params_data, and fetch_param_edit_data methods to use the new _fetch_api_data_by_key method instead of the removed _fetch_reg_key method. [1] [2] [3]
• custom_components/econet300/api.py: Updated the fetch_sys_params method to use the new _fetch_api_data_by_key method for fetching system parameters.