net: lib: http_server: add option to use ALPN

Web browsers don't support HTTP Upgrade mechanism to upgrade to HTTP2.
Instead, HTTP2 is supported only over TLS, and ALPN is used to negotiate
the protocol to be used.

This commit adds the supported HTTP protocols to the ALPN list, so that
web browsers can use HTTP2 with the server.

Signed-off-by: Matt Rodgers <mrodgers@witekio.com>