Support for Cloudfront as event source

I've got requests passing through a Cloudfront distribution directly to a lambda, no cache, passing all request headers with it. I'm using this approach because API Gateway and ALB are both overkill for my use-case, simply need a public endpoint with custom domain compatibility, hence Cloudfront.

Issue is that this library produces a `Unable to determine event source based on event` error in this setup. Having a look at the code it seems [here](https://github.com/vendia/serverless-express/blob/75cfbffeb3fd030695b9587a75300c7b8012ac6a/src/event-sources/utils.js#L70) that there is no conditional to detect this circumstance.

Since I'm passing all headers directly through Cloudfront, I assume that the event in this case looks a lot like it came directly from the client, rather than something triggered through another cloud service. This seems to me perhaps as a valid default case to design for in this library, in the absence of any other forwarders?