[Flight] Make it possible to use native gRPC Java APIs, and/or simplify the implementation

### Describe the enhancement requested

https://github.com/grpc/grpc-java/pull/10318 introduces a new type of blocking stub in grpc-java which handles streaming calls. This could drastically simplify DoGet/DoPut/DoExchange which try to present blocking interfaces to nonblocking APIs. However, preferably we would make it possible to use gRPC APIs directly instead of making users wait for us to wrap new APIs in even less documented new APIs. Also, it's not clear that this simplifies the server side at all (unfortunately).

Start by reviewing the grpc-java PR 10318 and the Flight DoGet, DoPut, and DoExchange interfaces described in the issue. Compare using the native blocking streaming APIs with the current wrappers, including the server-side implications. Done should mean a clearly chosen approach that either enables direct gRPC API use or demonstrably simplifies the implementation.

Written by the indexing model from the issue text.