reflection: export server types (#1043)

Makes it easier to store a reflection server in a variable
or get the gRPC service's name.

Author: zombiezen

tonic-reflection/src/server.rs

@@ -1,6 +1,6 @@
 use crate::proto::server_reflection_request::MessageRequest;
 use crate::proto::server_reflection_response::MessageResponse;
-use crate::proto::server_reflection_server::{ServerReflection, ServerReflectionServer};
+pub use crate::proto::server_reflection_server::{ServerReflection, ServerReflectionServer};
 use crate::proto::{
     FileDescriptorResponse, ListServiceResponse, ServerReflectionRequest, ServerReflectionResponse,
     ServiceResponse,