feat(apigatewayv2-integrations): stepfunctions integration (#28982)

### Issue

Closes #28904.

### Reason for this change

It is not possible to create an integration between Step Functions and HTTP API.

### Description of changes

You can create integration by `HttpStepFunctionsIntegration` class:

```ts
declare const httpApi: apigwv2.HttpApi;
declare const stateMachine: sfn.StateMachine;

const integration = new HttpStepFunctionIntegration('StepFunctionIntegration', {
	stateMachine,
  })

    httpApi.addRoutes({
      path: '/jobs',
      methods: [apigwv2.HttpMethod.POST],
      integration,
    });
```

### Description of how you validated changes

Added unit tests and integ test.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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

Author: badmintoncryer