Event definitions for AppSync Events

There are currently no event definitions for the relatively new AWS AppSync Event service. Going through the [AWS docs](https://docs.aws.amazon.com/appsync/latest/eventapi/event-api-welcome.html), I've collected the following list of events that could be added:

1. ~~[onPublish](https://docs.aws.amazon.com/appsync/latest/eventapi/channel-namespace-handlers.html#event-handler-onpublish)~~
2. ~~[onSubscribe](https://docs.aws.amazon.com/appsync/latest/eventapi/channel-namespace-handlers.html#event-handler-onpublish)~~
3. [Lambda authorization ](https://docs.aws.amazon.com/appsync/latest/eventapi/configure-event-api-auth.html#aws-lambda-authorization), consisting of a `authorizationRequest`, `authorizationContext` and a `authorizationReponse`.
4. Optionally/debatable definitions for the [event API websocket protocol messages](https://docs.aws.amazon.com/appsync/latest/eventapi/event-api-websocket-protocol.html#realtime-websocket-operations).

I'm open to adding these myself, since I'm currently developing a system that will use AppSync Event, so I need to write them regardless.

Should these definitions live in `aws_lambda_events::events::appsync` (potentially in a sub-module) or in a new module `aws_lambda_events::events::appsync_events` ?

Edit: I've just realized that `onPublish` and `onSubscribe` only supports javascript handlers - so scratch that...

Issue にリンクされている AWS AppSync Events のドキュメントを読み、その後、既存の aws_lambda_events::events::appsync モジュールを調べて、要求された定義との構成を比較します。Lambda 認証と websocket プロトコルメッセージが対象範囲に含まれるか、またそれらを appsync に置くべきかサブモジュールに置くべきかを判断します。合意された AppSync Event の定義が既存のイベントモジュールと一貫した形で表現されれば完了です。

索引モデルが issue の本文から書いたものです。