UNPIVOT - maintain original types and throw a better exception if type matching is not possible

[Mytherin]

Follow-up from #10115

UNPIVOT maintains the original input types again as it did previously. It also now requires types to be implicitly castable to one another, otherwise the UNPIVOT statement will fail:

UNPIVOT (SELECT 42 AS Jan, 'hello' as Feb) ON Jan, Feb;
-- Error: Binder Error: Cannot create a list of types INTEGER and VARCHAR - an explicit cast is required
-- Full list: Jan INTEGER, Feb VARCHAR
--                         ^
UNPIVOT (SELECT 42::VARCHAR AS Jan, 'hello' as Feb) ON Jan, Feb;
┌─────────┬─────────┐
│  name   │  value  │
│ varchar │ varchar │
├─────────┼─────────┤
│ Jan     │ 42      │
│ Feb     │ hello   │
└─────────┴─────────┘

This matches the behavior of both BigQuery and Snowflake.