feat: add request validator schema for http events

[paolorossig]

It adds support for http request validators, same behavior as for functions http events.

The serverless definition is as follows:

stepFunctions:
  stateMachines:
    First:
      events:
        - http:
            path: path/to
            method: POST
            request:
              schemas:
                application/json: start-execution
                  name:  TestSchema
                  schema: ${file(schema.json)}
      definition:
        StartAt: Test
        States:
          ...
    

Closes #554

[horike37]

@paolorossig
Thank you for this PR. Looks good so far. And as the rest of the tasks, we have to write documentation.
Would you add an explanation of this feature in the Readme so that people know how to use this?

[paolorossig]

@horike37 You are welcome! I just updated the README with the feature explanation of request validators with some examples.

[paolorossig]

@horike37 did you have a chance to review the missing documentation of this pr?

[horike37]

@paolorossig
Sorry for being late. LGTM!

[horike37]

🎉 This PR is included in version 3.15.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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