Added exchange interface.

src/interface/exchange.ts

@@ -0,0 +1,11 @@
+import { PortfolioProvider } from "./portfolio";
+import { QuoteProvider } from "./quote";
+
+export interface Exchange<Credentials> {
+    readonly credentials: Credentials;
+
+    readonly portfolioProvider: PortfolioProvider;
+    readonly quoteProvider: QuoteProvider;
+
+    readonly name: string;
+}