airframe-grpc: Server reflection support

[xerial]

https://github.com/grpc/grpc-java/blob/master/documentation/server-reflection-tutorial.md#enable-server-reflection

[xerial]

Although directly implementing the spec of gRPC reflection will require Protobuf, adding a support for similar functionality by returning the contents of Router would be possible.

For example, it would be interesting if we have a command-line client to read the list of RPC endpoints:

$ airframe ls http://localhost:8080 
.. list of RPC endpoints