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.
• 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

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.32%. Comparing base (512cf95) to head (bd13e87).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #32139      +/-   ##
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- Coverage   78.34%   78.32%   -0.02%     
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  Files         105      105              
  Lines        7170     7170              
  Branches     1315     1315              
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- Hits         5617     5616       -1     
- Misses       1365     1366       +1     
  Partials      188      188              

Flag	Coverage Δ
suite.unit	78.32% <ø> (-0.02%)	⬇️

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Components	Coverage Δ
packages/aws-cdk	78.32% <ø> (-0.02%)	⬇️

[MathieuGilbert]

Thanks for your help in getting these changes to green @ePak. I think it was the npm test after running the old tests that got it to pass. Unfortunately I don't get much time to work on this, so it's been dragging on.

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 94e9df4
• Result: SUCCEEDED
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