refactor: add PersistentConfigurationStorage interface

Author: rasendubi

src/persistent-configuration-storage.ts

@@ -0,0 +1,24 @@
+import { IConfiguration } from './i-configuration';
+
+/**
+ * Persistent configuration storages are responsible for persisting
+ * configuration between SDK reloads.
+ */
+// TODO: replace `IConfiguration` with a concrete `Configuration` type.
+export interface PersistentConfigurationStorage {
+  /**
+   * Load configuration from the persistent storage.
+   *
+   * The method may fail to load a configuration or throw an
+   * exception (which is generally ignored).
+   */
+  loadConfiguration(): PromiseLike<IConfiguration | null>;
+
+  /**
+   * Store configuration to the persistent storage.
+   *
+   * The method is allowed to do async work (which is not awaited) or
+   * throw exceptions (which are ignored).
+   */
+  storeConfiguration(configuration: IConfiguration | null): void;
+}