fix(stepfunctions-tasks): allow passing apiEndpoint as intrinsic function

[MathieuGilbert]

Issue #29925

Closes #29925, #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.
• A test stack was deployed with an API Gateway endpoint at GET /testing with basic auth Connection and was successfully invoked with task and payload { "endpointName: "testing" }:

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

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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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

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[MathieuGilbert]

Clarification Request

What is the process for updating integration tests? My previous attempts failed in CI due to snapshots not matching, which they shouldn't when the test changes.

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