Add support for VARIANT type

[wmTJc9IK0Q]

With duckdb/duckdb#19674 landing in upstream, we can start adding support for this new type to binding/appending and reads.

I will probably try to take a look at this soon if it is not already planned by DuckDB Labs.

[taniabogatsch]

Not planned by us ATM, no. So happy to review a PR! :)

[wmTJc9IK0Q]

Looked into this today. The C API doesn't currently expose the variant column type, it shows up as an invalid type right now.

[wmTJc9IK0Q]

Opened the C API PR here: duckdb/duckdb#20210