rhg: use default configitem in `cat` Now that we've declared the items, we can get rid of the manual fallback.

rhg: add config defaults to configitems.toml This will allow us to use the same default config mechanic as the rest of the code.

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.