[jmesnil]

Add a boolean management attribute enable-server-reflection (default: false, stability level: experimental) that, when set to true, registers ProtoReflectionService on the Netty gRPC server at startup to support gRPC server reflection.

• Add io.grpc:grpc-services dependency to subsystem and galleon-pack common
• Add enable-server-reflection attribute to GrpcSubsystemDefinition
• Conditionally register ProtoReflectionService in GrpcServerService.start()
• Update XSD schema with new attribute
• Update subsystem unit test fixtures
• Add NoServerReflectionTest: verifies reflection is disabled by default
• Add EnableServerReflectionTest: enables via management write-attribute + reload-enhanced, verifies helloworld.Greeter appears in reflection response

This fixes #493

[jmesnil]

@hpehl Once this and #490 are done, we could update the examples to leverage this new attribute so that the client will no longer need to have access to the proto file (that's simpler at least for prototyping with grpcurl).

The server started by the example would have to be provisioned and started at the experimental stability level with this attribute enabled:

./target/wildfly/bin/standalone.sh --stability experimental

and grpcurl can list, describe and serve the services:

$ grpcurl  -plaintext localhost:9555 list

grpc.reflection.v1.ServerReflection
helloworld.Greeter

$ grpcurl  -plaintext localhost:9555 describe  helloworld.Greeter

helloworld.Greeter is a service:
service Greeter {
  rpc SayHello ( .helloworld.HelloRequest ) returns ( .helloworld.HelloReply );
}

$ grpcurl -plaintext \
  -d '{"name":"Bob"}' \
  localhost:9555 helloworld.Greeter/SayHello

{
  "message": "Hello Bob"
}

[hpehl]

LGTM!