fix(apigatewayv2): some methods of the defaultStage are not availab…

…le without casting it to `IHttpStage` (#15607)

----

Closes #14787. Changes the defaultStage to an IHttpStage.

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-apigatewayv2/lib/http/api.ts

@@ -4,11 +4,11 @@ import { Construct } from 'constructs';
 import { CfnApi, CfnApiProps } from '../apigatewayv2.generated';
 import { IApi } from '../common/api';
 import { ApiBase } from '../common/base';
-import { DomainMappingOptions, IStage } from '../common/stage';
+import { DomainMappingOptions } from '../common/stage';
 import { IHttpRouteAuthorizer } from './authorizer';
 import { IHttpRouteIntegration, HttpIntegration, HttpRouteIntegrationConfig } from './integration';
 import { BatchHttpRouteOptions, HttpMethod, HttpRoute, HttpRouteKey } from './route';
-import { HttpStage, HttpStageOptions } from './stage';
+import { IHttpStage, HttpStage, HttpStageOptions } from './stage';
 import { VpcLink, VpcLinkProps } from './vpc-link';
 
 /**
@@ -366,7 +366,7 @@ export class HttpApi extends HttpApiBase {
   /**
    * The default stage of this API
    */
-  public readonly defaultStage: IStage | undefined;
+  public readonly defaultStage: IHttpStage | undefined;
 
   private readonly _apiEndpoint: string;