Add `allowUnsignedExtensions` to DuckDB Config

[Y--]

Right now one need two different builds if they want to switch between a version of DuckDB-WASM that accept unsigned extensions and one that doesn't. This is not consistent with other platforms (such as CLI which can be started with -unsigned or Python which can open a DB with allow_unsigned_extensions configuration)

This PR allows to open the DB with this flag as well.

[Mytherin]

Thanks!