Add SHA256 support for REST shared-secret auth

I'm building a webrtc solution and I'm using Kamailio auth_ephemeral module for shared secret client authentication which uses the same HMAC-SHA hash for ephemeral credentials as coturn's REST API shared-secret auth. However, Kamailio's implementation supports additional hash algorithms, not just SHA1. To allow for my webrtc client to use 1 ephemeral SHA256 hash to authenticate to both Kamailio and coturn, I have forked and created a patch to enable SHA256 support for REST shared-secret auth.

Please let me know if this contribution is useful and welcome. I will gladly create a PR to merge this functionality.

https://github.com/jschwinn/coturn/commits/shared-secret-sha256/

Thanks Jamie Schwinn