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.

accountsdb-plugin-manager/src/accountsdb_plugin_manager.rs

@@ -0,0 +1,55 @@
+/// Managing the AccountsDb plugins
+use {
+    libloading::{Library, Symbol},
+    log::*,
+    solana_accountsdb_plugin_interface::accountsdb_plugin_interface::AccountsDbPlugin,
+    std::error::Error,
+};
+
+#[derive(Default, Debug)]
+pub struct AccountsDbPluginManager {
+    pub plugins: Vec<Box<dyn AccountsDbPlugin>>,
+    libs: Vec<Library>,
+}
+
+impl AccountsDbPluginManager {
+    pub fn new() -> Self {
+        AccountsDbPluginManager {
+            plugins: Vec::default(),
+            libs: Vec::default(),
+        }
+    }
+
+    /// # Safety
+    ///
+    /// This function loads the dynamically linked library specified in the path. The library
+    /// must do necessary initializations.
+    pub unsafe fn load_plugin(
+        &mut self,
+        libpath: &str,
+        config_file: &str,
+    ) -> Result<(), Box<dyn Error>> {
+        type PluginConstructor = unsafe fn() -> *mut dyn AccountsDbPlugin;
+        let lib = Library::new(libpath)?;
+        let constructor: Symbol<PluginConstructor> = lib.get(b"_create_plugin")?;
+        let plugin_raw = constructor();
+        let mut plugin = Box::from_raw(plugin_raw);
+        plugin.on_load(config_file)?;
+        self.plugins.push(plugin);
+        self.libs.push(lib);
+        Ok(())
+    }
+
+    /// Unload all plugins and loaded plugin libraries, making sure to fire
+    /// their `on_plugin_unload()` methods so they can do any necessary cleanup.
+    pub fn unload(&mut self) {
+        for mut plugin in self.plugins.drain(..) {
+            info!("Unloading plugin for {:?}", plugin.name());
+            plugin.on_unload();
+        }
+
+        for lib in self.libs.drain(..) {
+            drop(lib);
+        }
+    }
+}