Support unencrypted HTTP/2 connections (i.e. h2c)

As seen in this forum question, previously the k6 docs indirectly mentioned support for h2c (HTTP/2 over cleartext TCP) connections. That was incorrect and I've fixed the docs to reflect that, but I'm also adding this issue so we can actually add support for it in the future. Please, add a :+1: here if you have a use case for h2c support in k6.

From a technical standpoint, we should likely do something like this. The problem is comes from the fact, that we rely on a preconfigured http.Transport from the Go stdlib, which we pass through http2.ConfigureTransport(transport) from golang.org/x/net/http2. We can't directly use the http2.Transport from the example above, because it doesn't contain a bunch of the options that http.Transport has and that we use...

Related issues:

• https://github.com/loadimpact/k6/issues/548
• https://github.com/loadimpact/k6/issues/936
• https://github.com/loadimpact/k6/issues/866