Add api config to customize API Gateway Responses

There is a need and use for customizing API Gateway Responses in Chalice. Specifically, I mean the ones you can currently edit in the console on this page:

So you can set custom status response codes, headers, and possibly templates.

I’d started #1270 to try implementing it, but @stealthycoin it would be good to discuss the implementation in an issue. In the pr I tried implementing it in the chalice config, but as was pointed out, it would be more appropriate for this to be managed on the app itself, e.g.

app = Chalice(app_name="helloworld")
app.api.<yet to be named> = <some config we haven't defined yet>

So some things we need to address in this issue are:

1. What should the name of the property on app.api be?
   • I personally like something like gateway_responses, but am open to other people’s input.
2. How should this configuration be defined? There is the option of JSON, which as pointed out in the pr, is not as friendly. @stealthycoin suggested using a dict to define it. Here is a link to the possible values that we could define: https://docs.aws.amazon.com/apigateway/api-reference/resource/gateway-response/#properties
   • I agree that this should be a dict, and we could make it a mapping of response types to config options. E.g.

     app.api.gateway_responses = {
         'default_4xx': {
             'response_parameters': {
                 'access-control-allow-origin': "'*'"
             }
         } 
     }
     

     Maybe this is too verbose? I’d be happy to get some feedback or alternative options.

[felipe-lee]

Forgot to say that one of the big reasons for being able to do this is so that you can set up Default 4XX and 5XX response parameters for CORS setup.

[alexevansigg]

+1 for this requirement.

In my scenario i’m using a customer Lambda Authoriser… if the lambda authorizor rejects the users then it will trigger the default API responses, and without a means to customize the Response template, there’s no means to return meaningful response details.