node-fetch-server: http2 support

[eugene1g]

Hi @mjackson ,

I'd like node-fetch-server to support node:http2 - would you be open to a PR for this, or is the scope now fixed?

[mjackson]

I'd be open to supporting http/2, but honestly I don't have a lot of experience building h2 servers. What do you think a high-level API might look like?

[eugene1g]

Thanks! The API will be the same, perhaps even using createRequestListener for both HTTP1 and HTTP2.

I built another library to do this 1-2 years ago and recently thought about open open-sourcing it, then found node-fetch-server. You and I used more or less the same strategy, and the libraries have nearly identical performance, so the only thing missing from node-fetch-server is HTTP2 support. Might as well merge my work into this.

I'll put together a PR with tests for you to look over.