Add CORS support (Serverless v1)

[patoncrispy]

Feature Request:

I'd like to enable CORS support for API Gateway endpoints in Serverless v1. A schema example could look like this:

functions: 
  user:
    handler: handler.user
    events:
      - http:
          method: post
          cors: true
custom: 
    cors:
        allow-origin: '*'
        allow-headers:
            - Content-Type
            - X-Amz-Date
            - Authorization
            - X-Api-Key
        allow-methods:
            - POST
plugins:
    - api-gateway-cors-plugin

When run (with deployment), it will create the OPTIONS method and the appropriate 200 responses for specified methods, adding the specified origin and headers. Headers, Methods and Origin are required headers by API Gateway, so I reckon a warning should be printed letting the user know what they need to specify. It could also display other validation warnings, such as having wildcard (*) header/origin settings, or if a setting isn't supported by API Gateway, etc., though it would probably be good to have an option to suppress warnings for specific stages.

Benefits:

• It will enable easier development of single page apps.
• We can have CORS support for API Gateway. Yay!

Drawbacks:

• None that I can think of...

[patoncrispy]

Would it be possible for me to implement this feature? I'd like to start contributing to Serverless in some way, so if not this task then maybe something else?

[austencollins]

@patoncrispy Absolutely :) You're fantastic, @patoncrispy. We read all of your posts. /cc @eahefnawy could he get started on this?

[patoncrispy]

@ac360 :D Thanks so much! No pressure now! :)

[codepreneur]

cant wait! 👍

[eahefnawy]

@patoncrispy definitely and thanks for that! We were just thinking about that and discussing it in #1596 . Let's move the discussion there 😊.

[eahefnawy]

Closing this since we're now tracking this in Issue 1596