Introduce a plugin system for picodata cluster.
Plugin system contains:
- `picoplugin` crate with plugin interfaces.
This crate exposes plugin interfaces for user implementation and API
that used by picodata to interact with plugin as with a code from dynamic library.
- Support for plugin work cycle in picodata instance work cycle.
Picodata instance interacts with running plugin by `PluginManager`
component.
Typically, plugins react to events that occur in instance or whole cluster.
- New lua API with plugin CRUD actions.
    - pico.create_plugin - load a new plugin into picodata cluster
    - pico.remove_plugin - remove plugin from picodata cluster
    - pico.update_plugin_config - update plugin service configuration