Support Request Validation API Gateway Feature

This is a Feature Proposal

Description

The API Gateway Request Validation is now supported by Serverless framework:

functions:
  create-something:
    handler: functions/create-something.handler
    events:
      - http:
          path: ${self:custom.version}/things
          method: post
          request:
            schema:
              application/json: ${file(thing.schema.json)}

However, seems the plugin does not support when trying to add the same configuration inside of an event:

stepFunctions:
  stateMachines:
    create-things:
      events:
        - http:
            path: ${self:custom.version}/things
            method: post
            request:
              schema:
                application/json: ${file(thing.schema.json)}

Additional Data

• Serverless Framework Core Version you're using: latest
• The Plugin Version you're using: latest
• Operating System: N/A
• Stack Trace: N/A
• Provider Error messages: N/A

Start by reproducing the configuration in the issue with a Step Functions state machine HTTP event and compare it with the working Serverless Framework function event. Trace the plugin's event handling and existing API Gateway tests, if present. Done means the request schema configuration is accepted for the state machine event and produces the expected API Gateway validation behavior.