Add support for MariaDB ENUM columns as pandas Categorical

[MichaelCurrie]

Currently ENUM columns generally come to us as type_code STRING (instead of the usual type_code VAR_STRING). And we convert STRING to Pandas "string" (nullable str) type. But converting ENUM to a proper Pandas Categorical would be nice:

• Detect ENUM columns from SQL schema
• Extract enum values from column definitions
• Convert to pandas Categorical dtype for better performance and type safety
• Handle round-trip conversion (Categorical → string for insertion)

This would be a nice enhancement that leverages pandas' categorical functionality for better memory usage and type safety with enum data! 🎯