Custom Checks API and Completion of 0.8.0

[patelheet30]

Added

• Custom Check API: Users can now register their own custom checks using the register_check method on the lint accessor. Custom checks should be functions that accept a DataFrame and return a list of warning strings.
• Support for custom checks in the main report() method, allowing users to include their custom checks in the data quality audit.
• Users can also unregister custom checks using the unregister_check method on the lint accessor.