Add RestAPIId field in the API Gateway Authorizer struct

[DrFaust92]

Feature description

Add a RestAPIId field in the Authorizer struct
https://github.com/aws/aws-sdk-go/blob/master/service/apigateway/api.go#L11595

i can open a PR for the API side but i can't change the model per the contribution guide (i also assume there is backend component for that)

I tried working on hashicorp/terraform-provider-aws#9529 but i couldnt think of a soultion without having this field.

Describe alternatives you've considered

List all apigateways and look at each authorizer and find the right api