Add lambda at edge support

[danielcondemarin]

Adds support for CloudFront lambda event triggers:

For example:

# serverless.yml

distribution:
  component: '@serverless/aws-cloudfront'
  inputs:
    origins:
      - url: https://sampleorigin.com
        pathPatterns:
          /sample/path:
            ttl: 10
            lambda@edge:
              viewer-request: arn:aws:lambda:us-east-1:123:function:myFunc:5 # lambda ARN including version

Event types supported: viewer-request | origin-request | viewer-response | origin-response

https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/lambda-cloudfront-trigger-events.html

In future you may want to consider provisioning the Lambda as part of the aws-cloudfront component. That could be another iteration of the API, for example by setting an object rather than the ARN string:

lambda@edge:
     viewer-request:
       code: ./edge-src
       handler: ./my-handler.js

[eahefnawy]

Thanks @danielcondemarin ... I've added some comments

[eahefnawy]

is the version a strict requirement for Lambda@Edge? I don't think we support versioning in our lambda component 🤔 ... In that case, this component wouldn't be composable with a lambda/function component cause they output a simple versionless ARN.

This effectively means that users of this component could only use it with an already deployed lambda via another tool, which kinda defeats the purpose of the whole thing. What you think? 🤔

[danielcondemarin]

Unfortunately it must be the ARN of a function version and $LATEST can't be used either. How difficult would it be to add versioning onto the aws-lambda component? Or maybe a higher level component that uses aws-lambda but also manages versioning?