AccountsDb plugin framework (#20047)

Summary of Changes

Create a plugin mechanism in the accounts update path so that accounts data can be streamed out to external data stores (be it Kafka or Postgres). The plugin mechanism allows

Data stores of connection strings/credentials to be configured,
Accounts with patterns to be streamed
PostgreSQL implementation of the streaming for different destination stores to be plugged in.

The code comprises 4 major parts:

accountsdb-plugin-intf: defines the plugin interface which concrete plugin should implement.
accountsdb-plugin-manager: manages the load/unload of plugins and provide interfaces which the validator can notify of accounts update to plugins.
accountsdb-plugin-postgres: the concrete plugin implementation for PostgreSQL
The validator integrations: updated streamed right after snapshot restore and after account update from transaction processing or other real updates.
The plugin is optionally loaded on demand by new validator CLI argument -- there is no impact if the plugin is not loaded.

runtime/src/accounts_update_notifier_interface.rs

@@ -0,0 +1,25 @@
+use {
+    crate::append_vec::StoredAccountMeta,
+    solana_sdk::{account::AccountSharedData, clock::Slot, pubkey::Pubkey},
+    std::sync::{Arc, RwLock},
+};
+
+pub trait AccountsUpdateNotifierInterface: std::fmt::Debug {
+    /// Notified when an account is updated at runtime, due to transaction activities
+    fn notify_account_update(&self, slot: Slot, pubkey: &Pubkey, account: &AccountSharedData);
+
+    /// Notified when the AccountsDb is initialized at start when restored
+    /// from a snapshot.
+    fn notify_account_restore_from_snapshot(&self, slot: Slot, account: &StoredAccountMeta);
+
+    /// Notified when a slot is optimistically confirmed
+    fn notify_slot_confirmed(&self, slot: Slot, parent: Option<Slot>);
+
+    /// Notified when a slot is marked frozen.
+    fn notify_slot_processed(&self, slot: Slot, parent: Option<Slot>);
+
+    /// Notified when a slot is rooted.
+    fn notify_slot_rooted(&self, slot: Slot, parent: Option<Slot>);
+}
+
+pub type AccountsUpdateNotifier = Arc<RwLock<dyn AccountsUpdateNotifierInterface + Sync + Send>>;