Change method of using Hapi RouteOptionsApp interface

In my last commit I was extending this interface -- however, that
could conceivably cause issues in the future for other areas in the
project that need to use the route "app" options. Instead, now I am
using TypeScript "Declaration Merging" like other Kibana components
that need to interact with Hapi do.

src/core/server/http/router/request.ts

@@ -18,7 +18,7 @@
  */
 
 import { Url } from 'url';
-import { Request, RouteOptionsApp } from 'hapi';
+import { Request } from 'hapi';
 
 import { ObjectType, TypeOf } from '@kbn/config-schema';
 
@@ -150,7 +150,7 @@ export class KibanaRequest<Params = unknown, Query = unknown, Body = unknown> {
 
  private getRouteInfo() {
  const request = this[requestSymbol];
- const app = request.route.settings.app as KibanaRouteOptionsApp;
+ const app = request.route.settings.app;
  return {
  path: request.path,
  method: request.method,
@@ -189,11 +189,3 @@ function isRequest(request: any): request is LegacyRequest {
 export function isRealRequest(request: unknown): request is KibanaRequest | LegacyRequest {
  return isKibanaRequest(request) || isRequest(request);
 }
-
-/**
- * Extend the Hapi interface for application-specific configuration on routes
- * @internal
- */
-interface KibanaRouteOptionsApp extends RouteOptionsApp {
- extendsSession: boolean;
-}

src/legacy/server/kbn_server.d.ts

@@ -97,6 +97,14 @@ declare module 'hapi' {
  interface ResponseToolkit {
  renderAppWithDefaultConfig(app: UiApp): ResponseObject;
  }
+
+ /**
+ * Extend the Hapi interface for application-specific configuration on routes
+ * @internal
+ */
+ interface RouteOptionsApp {
+ extendsSession: boolean;
+ }
 }
 
 type KbnMixinFunc = (kbnServer: KbnServer, server: Server, config: any) => Promise<any> | void;