feat(sql): made DbInstances public for managing directly from Rust

[thewh1teagle]

This PR expands the functionality of the SQL plugin in the plugins-workspace for v2 by exposing database instances for direct Rust access.

Example useage:

use tauri_plugin_sql::DbInstances;

let instances = app_handle.state::<DbInstances>();
let instances = instances.inner().0.lock().await;
println!("instances: {:?}", instances);

[danielniccoli]

Hope this will be merged soon :)

[smurthy]

This feature would be useful.