Preserve original enum value case on retrieval

[fulghum]

When retrieving enum values for ShowCreateTable,Information_Schema.Columns, and Describe/ShowColumns, they were always being converted to lowercase, which doesn't match MySQL's behavior. Instead, we should preserve the case each enum value was defined with, whenever we return those values.

This did not affect enum values returned as part of table data – these values were already correctly preserving the originally defined case.

Fixes: https://github.com/dolthub/dolt/issues/3991 Fixes: https://github.com/dolthub/go-mysql-server/issues/1161