[Chronik] Add PluginsWriter and PluginsReader (only for PluginMeta)

Summary:
Add a minimal implementation for PluginsWriter and PluginsReader that only deals with PluginMeta to store the state of plugins.

This doesn't affect the behavoir of Chronik yet.

Test Plan: cargo test -p chronik-db --lib test_plugin_metas

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16520