Ability to define apiKeySource for ApiGateway with custom authorizer

[sandor11]

This is a Feature Proposal

Description

I would like to be able to provide the ability to have my customer authorizer be the source of the api key, as per the aws docs.

https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-lambda-authorizer-output.html
https://docs.aws.amazon.com/apigateway/api-reference/resource/rest-api/#apiKeySource

Event though there is no reference to this setting in the serverless docs, I have tried setting the value in the global provider settings

provider:
    apiGateway:
        apiKeySource: AUTHORIZER

However the cloud formation template does not produce any result with this, which was kind of expected.

"ApiGatewayRestApi": {
    "Type": "AWS::ApiGateway::RestApi",
    "Properties": {
        "Name": "test-api-gateway-dev",
        "EndpointConfiguration": {
            "Types": [
                "REGIONAL"
            ]
        }
    }
}

This should be technically possible, I am wondering if there is any support for this? or if serverless enforces the use of the HEADER value which then requires the key to be provided int the x-api-key header.

• The use case
  I have the need to expose an API gateway endpoint, to a third party. I want the endpoint to remain private, but I do not want to give them the AWS API Gateway Key. Instead, I want to be able to use a custom authoriser, which will be the source of the key.

This will allow me to keep the benefits of a usage plan, without exposing the details of the key.

• If there is additional config how would it look
  In terms of config, what I specified above, could be an option. Alternatively, it could be done the same way as the endpointType directly under the provider.

The resulting cloud formation may be trivial https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-apigateway-restapi.html#cfn-apigateway-restapi-apikeysourcetype

"ApiGatewayRestApi": {
    "Type": "AWS::ApiGateway::RestApi",
    "Properties": {
        "Name": "test-api-gateway-dev",
        "EndpointConfiguration": {
            "Types": [
                "REGIONAL"
            ]
        },
        "ApiKeySourceType": "AUTHORIZER"
    }
}

I have not been able to find a reference to this in the serverless docs, forums or the codebase. I would also be happy to work on this if this is something you would like to include.

Additional Data

• Serverless Framework Version: 1.27.3
• Operating System: macOS 10.13.4