APIGW struct missing fields?

**Is your feature request related to a problem? Please describe.**

Hello! The websocket API request event context type [APIGatewayWebsocketProxyRequestContext](https://github.com/aws/aws-lambda-go/blob/main/events/apigw.go#L168) does not have fields for `disconnectStatusCode` and `disconnectReason` that are populated when the `$disconnect` route is hit, so this information is not available to the route integration.


**Describe the solution you'd like**
The fields added to the struct so the data is available.

Thank you!

Start at events/apigw.go around APIGatewayWebsocketProxyRequestContext and compare the $disconnect event data described in the issue. Add fields for disconnectStatusCode and disconnectReason so the values are available through the context passed to the route integration; done means both fields are represented in the struct.

Written by the indexing model from the issue text.