Converting alert type and app id to camel case in the CloudEvent

[colerogers]

Fixes a bug where the SDK expected camel cased fields, but eventarc did not, so we convert them before passing the event to the end users code.

[marcgovi]

Adding a note here that CloudEvent specs explicitly mention that extension context attributes (eg: appid and alerttype for Firebase Alerts) must be lowercase https://github.com/cloudevents/spec/blob/v1.0.2/cloudevents/spec.md#extension-context-attributes.

I'm worried that the firebase functions is converting them to camel case format instead before delivering the CloudEvent object to the customer function.

[colerogers]

@marcgovi We had a discussion offline, but posting here for visibility.

Basically, we try to match the TS/JS norms in the Functions SDK to give the best possible developer experience. In this case, converting fields to camelcase in the event signature. We have a history of changing the event with other integrations too, for example with RTDB, we convert fields to camelcase and also create a custom ref object so customers don't have to import the Admin SDK directly. We think these quality of life improvements make a difference to developers and should be included in the SDK