Allow configuring of DuckDB settings

[JelteF]

DuckDB has a bunch of settings that users probably want to tune based on their specific usage. At least threads memory usage should be configurable, and probably more. I think it is sensible to use postgres its GUC system for this, which would allow people to put things like the following in their postgresql.conf file or use SET commands:

duckdb.threads = 1
duckdb.memory = 256MB
duckdb.disabled_filesystems = 'LocalFilesystem'
duckdb.enable_external_access = true

Part of this task is figuring out which duckdb settings make sense for people to configure.