add REST API support

[spencersevilla]

We could greatly use a simple/basic way to query the EPC for various stats/information. At a minimum, this should include everything in the stats printout (number of eNB/UE) as well as uptime and some other basic health stats.

[spencersevilla]

we can use libonion to do this https://www.coralbits.com/libonion/

[matt9j]

Another option is to use an RPC style framework like grpc w/ protobuf. It's more structured so there is a little more work up front to specify messages, but gives you nice type checked abstractions into other supported languages instead of just a uri. RPC is likely overkill for simple REST stats querying, but does scale better if we do ever want to stream information out or notify on events.