Refactor slot status notification to decouple from accounts notifications (#21308)

Problem

Slot status can be used of in other scenarios in addition to account information such as transactions, blocks. The current implementation is too tightly coupled.

Summary of Changes

Decouple the slot status notification from accounts notification. Created a new slot status notification module.

accountsdb-plugin-manager/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod accounts_update_notifier;
 pub mod accountsdb_plugin_manager;
 pub mod accountsdb_plugin_service;
+pub mod slot_status_notifier;
 pub mod slot_status_observer;