configitems: move blackbox's config items to the new configitems.toml
In order for the Rust code to gain access to default values of in-core
extensions that have a Rust implementation, we need to centralize them
alongside the core items declarations.
This is the first and so far only one of the extensions that have gained
Rust support, I don't think it's worth the churn to move the rest of the
extension's configitems yet.