fastify support

In [this video](https://www.youtube.com/watch?v=LOeioOKUKI8), firebase functions can successfully integrate with node.js application, specifically express framework. I want to use [fastify](https://www.fastify.io/benchmarks/) framework because it has better performance than express.

Unfortunately, when I am using fastify, I got error like this:
```js
TypeError: handler is not a function
at cloudFunction (/srv/node_modules/firebase-functions/lib/providers/https.js:49:9)
at /worker/worker.js:783:7
at /worker/worker.js:766:11
at _combinedTickCallback (internal/process/next_tick.js:132:7)
at process._tickDomainCallback (internal/process/next_tick.js:219:9)
```