rust-blackbox: use `is_extension_enabled` config helper It's there, may as well use it to make the code clearer and less bug-prone.

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.

rust-config: add config getters that don't fall back to defaults This is useful in cases where we access config items that are more... lenient with their types than a fresh new system would allow. For now there is only a single use of this, but we might get more later.

rust-config: add support for default config items Now that configitems.toml exists, we can read from it the default values for all core config items. We will add the devel-warning for use of undeclared config items in a later patch when we're done adding the missing entries for `rhg`.