Currently, using chronik-plugin in the indexer is a bit tedious, as everything has to be #[cfg(feature = "plugins")] gated, which is hard to maintain.
Instead, we move the current chronik-plugin to chronik-plugin-impl, which is then re-exported in a new chronik-plugin crate if the plugins feature is enabled, and otherwise it exports fallback structs.
Depends on D15394.