fix: update http event definition for step functions

[paolorossig]

Overview

• Description: http events have other properties supported by the plugin others than just path
• Schema update type: extend AwsStateMachineHttpEvent definition
• Services affected: serverless-step-functions plugin (step-functions)

References

• [documentation/forum thread links]

How was this tested?

[Describe what steps were taken to ensure this schema change is correct & necessary]