fix(stepfunctions-tasks): allow passing apiEndpoint as intrinsic function (under feature flag) (aws#32139)

### Issue aws#29925

Closes aws#29925, aws#30749.

### Reason for this change

The `apiEndpoint` prop currently only works when it's a string (ie. `TaskInput.fromText('some/text')`), with the task failing when passed as a reference (ie. `TaskInput.fromText(JsonPath.format('some/text/{}', '123')`). This is needed to allow for dynamic parts in the path.

### Description of changes

- Change the `ApiEndpoint` task parameter to use the `JsonPath.format` intrinsic function to combine the `apiRoot` and `apiEndpoint` props, instead of basic string concatenation.
- Update README entry with more complex example.

### Description of how you validated changes

- A unit test was added to cover passing formatted input.
- An integration test was added using `fromJsonPathAt` for the endpoint.
- A test stack was deployed with an API Gateway endpoint with basic auth Connection and was successfully invoked with dynamic payload:

```
    const httpInvokeTask = new HttpInvoke(this, 'HttpInvoke', {
      apiRoot: api.url,
      apiEndpoint: TaskInput.fromJsonPathAt('$.endpointName'),
      method: TaskInput.fromText('GET'),
      connection,
      outputPath: '$.ResponseBody',
    })
```

### 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: MathieuGilbert