core: support ALPN (HTTP/1.1 HTTP/2 negotiation)

As of https://github.com/lyft/envoy-mobile/pull/417, Envoy Mobile defaults traffic to HTTP/2. Depending on the server being communicated with, however, this can potentially cause problems with ALPN if the upstream server doesn't support HTTP/2. To resolve this, Envoy Mobile should be capable of doing this negotiation between HTTP/1.1 and HTTP/2.

There's an upstream issue in Envoy tracking adding support for this: https://github.com/envoyproxy/envoy/issues/3431.

Once support is added, the demo apps / liveliness CI tests should be updated to test both HTTP/1.1 and HTTP/2.