ExplanationConfig

Language-independent config for a per-case rtemis explanation. Mirrors the `ExplanationConfig` object: a kind of explanation plus its kind-specific settings. The kind is tagged rather than inferred, so a second kind of explanation can be added without changing how the first is read. `type` selects the variant and its settings are siblings of it, validated against the variant's own schema below. Anything not set takes rtemis's default, so an object holding only `type` is that variant with every default.