Change SocksToRtc to have a freedom-compatible interface.

This change also adds a manifest.json for SocksToRtc, but this
manifest is not yet used anywhere or verified by the build.
In fact, the previous annotation was incorrect.  When CHURN is enabled, this message is not a SignallingMessage.  However, typescript's typechecking is not strict enough to detect this typing error.

I have not cleaned up every instance of confused typing, but I have tried to remove them from lines that I was already editing.