Feature request: configure API gateway extensions in SAM template

## Description:

When using Swagger/Open API specifications to configure an API Gateway, you have to add your own [API Gateway Swagger Extensions](https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-swagger-extensions.html). This means your API specification is tainted with AWS-specific data. This is a feature request for some way of configuring functions and APIs from the SAM template and have it implicitly annotate the Open API specification.

### Example

Extend the [`Api` event source type ](https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api) so that you can map the request path and method to a particular Lambda invocation. This would generate the `x-amazon-apigateway-integration` field in the Open API specification implicitly when deploying. Example configuration (the `Extensions` field onwards):

```yaml
Resources:
HelloWorldFunction:
Type: AWS::Serverless::Function
Properties:
CodeUri: hello-world/
Handler: app.lambdaHandler
Runtime: nodejs8.10
Events:
HelloWorld:
Type: Api
Properties:
Path: /hello
Method: get
Extensions:
IntegrationResponse:
uri: Fn::Sub: arn:aws:apigateway:${AWS::Region}:lambda:path/2015-03-31/functions/${HelloWorldFunction.Arn}/invocations # This may also be implicit?
contentHandling: "CONVERT_TO_TEXT"
```

### Rationale

The idea here is to keep your Open API specification pure and free from AWS-specific data. This should make it much easier to configure the relationship between your Lambdas and API Gateway, because you only have to add configuration in the SAM template.