[Chronik] Move chronik-plugin to chronik-plugin-impl, add chronik-plugin multiplex crate

Summary:
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.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15395