Add gRPC reflection support to arrow flight

Addresses apache#1706, adding a
constant to the gRPC generated code so it is possible to include the
Flight and Flight SQL APIs in a `tonic-reflection` service.

Testing:

```
grpcurl --plaintext localhost:8082 list
```

arrow-flight/src/lib.rs

@@ -75,7 +75,6 @@ mod gen {
 pub mod flight_descriptor {
     use super::gen;
     pub use gen::flight_descriptor::DescriptorType;
-
     pub use gen::FILE_DESCRIPTOR_SET;
 }