Async support

Would it make sense to support async functions here, so that the event handlers could be called in an async manner and consequently call async libraries?

This way a single instance could be handling more requests in parallel, for example if the cloud function calls several external APIs in a sequence to complete a process, and spends most of the time just waiting for responses, and uses only a little memory and CPU in general.

It is possible to call async code from the current sync functions, but it does not help with scalability, as the cloud function itself still blocks during the whole operation. This can help to do things in parallel in a cloud function, but not with sequential operations, which are common at least in our use case.

The original JS runtime is async, like all things Node, but also Python has standard async in version 3. Consequently, firebase-admin-python has worked on adding async support, since google-auth added async support in 2021. https://github.com/firebase/firebase-admin-python/issues/104

> Hey Folks, We just released [v5.3.0](https://github.com/firebase/firebase-admin-python/releases/tag/v5.3.0) which added async support for the Cloud Firestore API. This is the first step into fulfilling this FR so please give it a try and let us know what you think!

Starlette is a popular, lightweight ASGI framework which could maybe used for this, the same way Flask is for the sync version? https://www.starlette.io/

No source files, tests, or entry points are named. Start by mapping the current synchronous event-handler entry points and reviewing the cited Starlette and firebase-admin-python async work. Done requires a defined async handler model, its runtime integration, and documented coverage for the sequential and concurrent use cases described.

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