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...

Leggi la documentazione di AWS AppSync Events collegata nell’issue, quindi esamina il modulo esistente aws_lambda_events::events::appsync per confrontarne l’organizzazione con le definizioni richieste. Determina se l’autorizzazione Lambda e i messaggi del protocollo websocket rientrano nell’ambito e se devono appartenere a appsync o a un sottomodulo. Il lavoro è completato quando le definizioni concordate di AppSync Event sono rappresentate in modo coerente con i moduli degli eventi esistenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.