Add strict checking for toml keys in blueprints. This allows us
to error early if there are unknown keys in a toml blueprint and
helps our users by spotting e.g. typos early.
This is similar to
https://github.com/osbuild/bootc-image-builder/pull/549
(thanks Ondrej!).
This commit provides a `blueprintload` package that can be used
to load blueprints from json/toml from a path. This will be used
in `bootc-image-builder` and `image-builder-cli` and should
eventually be merged into `images`.
