Create apigateway permissions from openapi definitions

### Describe your idea/feature/enhancement

Hey team, recently tried building out a rest API using `AWS::Serverless::Api` and openapi via DefinitionBody. It's great having my OpenAPI schemas converted to API gateway models and validation 💯 .

The only thing that's missing would be to have **apigw permissions automatically created.**

For example, the `Events` property on `AWS::Serverless::Function` automatically creates permissions for apigw to invoke the lambda.

The OpenAPI schema already defines the lambda handler via `x-amazon-apigateway-integration`, so it should be possible to create this permission in the background.

```yml
x-amazon-apigateway-integration:
uri:
Fn::Sub: arn:aws:apigateway:${AWS::Region}:lambda:path/2015-03-31/functions/${MyFunction.Arn}/invocations
```

### Proposal

Parse the DefinitionBody of `AWS::Serverless::Api` and create a `AWS::Lambda::Permission` resource for the attached lambda function.

Things to consider:
1. This requires no changes to the [SAM Spec](https://github.com/awslabs/serverless-application-model)