Support v1 of server reflection API

Now that [the server reflection schema is stable](https://github.com/grpc/grpc-proto/blob/master/grpc/reflection/v1/reflection.proto), it'd be great if `grpcurl` supported `v1` in addition to `v1alpha`. The new proto schema is identical to the old one, so this should be as simple as trying the `v1` URL and falling back to `v1alpha` if necessary — the remainder of the code can stay unchanged.

It looks like this is blocked on `grpc-go` shipping an equivalent of `google.golang.org/grpc/reflection/grpc_reflection_v1alpha`. Is that accurate, or could we make this change independently?

Start by checking grpc-go for the equivalent of google.golang.org/grpc/reflection/grpc_reflection_v1alpha and compare the v1 and v1alpha reflection URLs described in the issue. Confirm whether grpcurl can try v1 and fall back to v1alpha without other changes; done means v1 servers work while existing v1alpha behavior remains intact.

Written by the indexing model from the issue text.