feat: add performance optimisations for large datasets

[patelheet30]

Changed

• Optimised performance of check_duplicate_rows to handle large DataFrames with many rows more efficiently.
• Optimised performance of check_mixed_types using vectorised operations to reduce processing time on large DataFrames.
• Optimised performance of check_whitespace by leveraging pandas string methods for faster detection of whitespace issues.
• Optimised performance of check_outliers with reduced memory usage/allocation.