server: enable H2C for HTTP listeners via CLI flag

This follows the docs provided by the [github issue](1),
https://www.mailgun.com/blog/http-2-cleartext-h2c-client-example-go/

For manual testing, ensure that you have a curl version with the proper
"Features", as can be read off `curl --version`:

    curl 7.72.0 (x86_64-apple-darwin19.5.0) libcurl/7.72.0 OpenSSL/1.1.1g zlib/1.2.11 brotli/1.0.9 zstd/1.4.5 c-ares/1.16.1 libssh2/1.9.0 nghttp2/1.41.0 librtmp/2.3
    Release-Date: 2020-08-19
    Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp
    Features: AsynchDNS brotli GSS-API HTTP2 HTTPS-proxy IPv6 Kerberos Largefile libz Metalink NTLM NTLM_WB SPNEGO SSL TLS-SRP UnixSockets zstd

As described in the mailgun blog post, curl-openssl on homebrew has it for osx.

With this change, and started with

    ./opa_darwin_amd64 run -s --h2c --diagnostic-addr :8182

Both the ALPN and the "prior knowledge" modes work against the insecure endpoints:

    $ curl -v --http2 http://127.0.0.1:8181/metrics >/dev/null
    *   Trying 127.0.0.1:8181...
    > GET /metrics HTTP/1.1
    > Host: 127.0.0.1:8181
    > User-Agent: curl/7.72.0
    > Accept: */*
    > Connection: Upgrade, HTTP2-Settings
    > Upgrade: h2c
    > HTTP2-Settings: AAMAAABkAAQCAAAAAAIAAAAA
    >
    * Mark bundle as not supporting multiuse
    < HTTP/1.1 101 Switching Protocols
    < Connection: Upgrade
    < Upgrade: h2c
    * Received 101
    * Using HTTP2, server supports multi-use
    * Connection state changed (HTTP/2 confirmed)
    * Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
    * Connection state changed (MAX_CONCURRENT_STREAMS == 250)!
    < HTTP/2 200
    < content-type: text/plain; version=0.0.4; charset=utf-8
    < date: Wed, 30 Sep 2020 12:15:08 GMT
    <
    { [4096 bytes data]
    * Connection #0 to host 127.0.0.1 left intact
    $ curl -v --http2-prior-knowledge http://127.0.0.1:8181/metrics >/dev/null
    *   Trying 127.0.0.1:8181...
    * Using HTTP2, server supports multi-use
    * Connection state changed (HTTP/2 confirmed)
    * Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
    * Using Stream ID: 1 (easy handle 0x7f85c3814c00)
    > GET /metrics HTTP/2
    > Host: 127.0.0.1:8181
    > user-agent: curl/7.72.0
    > accept: */*
    >
    * Connection state changed (MAX_CONCURRENT_STREAMS == 250)!
    < HTTP/2 200
    < content-type: text/plain; version=0.0.4; charset=utf-8
    < date: Wed, 30 Sep 2020 12:15:13 GMT
    <
    { [4096 bytes data]
    * Connection #0 to host 127.0.0.1 left intact

[1]: https://github.com/open-policy-agent/opa/issues/2399

Signed-off-by: Stephan Renatus <srenatus@chef.io>