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GH-55396 : [C++][FlightRPC] Add async gRPC server - #55419
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Serve Flight RPCs on gRPC's generic callback API so handlers return arrow::Future and never occupy a worker thread while waiting. Motivation: the synchronous transport runs every RPC on a gRPC worker thread that the handler occupies until it returns, so throughput is bounded by the thread pool (which grows at most ~1 thread/second) rather than by the work in flight. Clients already have an async path; servers had none. API (server_async.h, flight_data_decoder.h): - AsyncGenericFlightServerBase: *Async handlers returning arrow::Future; unoverridden handlers answer NotImplemented. - AsyncFlightDataStream / AsyncFlightListing / AsyncResultStream: pull-based iterators, one item requested and written at a time. - AsyncFlightMessageReader / AsyncFlightMessageWriter: DoExchange halves. - AsyncFlightDataListener / AsyncFlightDataWriter: push-based DoPut, with PutResult metadata sendable from any thread. Transport (transport/grpc/async_grpc_*.cc): - One translation unit per part of the transport: service dispatch, DoGet, DoPut, DoExchange (reader, writer and reactor), Handshake, the streaming trio and the unary trio. - AsyncGenericFlightService creates one reactor per RPC from the method name; unknown methods get a fallback reactor answering UNIMPLEMENTED, so the typed sync service is not registered alongside it. - DoGet is served on a write-only bidi reactor: the generic callback API has no server-streaming reactor. - Reactors refcount themselves, since gRPC's callback API has no holds; write buffers are reactor members because StartWrite requires them to outlive the call. - DoPut serializes its PutResult writes: WriteMetadata() runs on the listener's thread while OnReadDone()/OnWriteDone() run on gRPC callback threads, and a half-close arriving mid-write now waits for the slot instead of starting a second write on the same buffer. - AsyncCallContext carries the call context; middleware and auth go through the shared GrpcServerCallContextHelper. Fixes found while building this: - Signal teardown ordering: Serve() published the server as the signal target before its self-pipe and shutdown callback were initialized, and did not restore handlers when it failed partway. - SIGPIPE: waking the signal thread with SelfPipe::Shutdown() closed the read end, so a handler entering just before that killed the process; wake with a plain payload instead. - Shutdown() on an uninitialized server cleared the process-wide signal target, disabling signal shutdown for another serving instance. Serve(), Shutdown() and Wait() now validate before entering the signal helper. - Sync/async lifecycle duplication: Init/Serve/Shutdown/Wait/port/location moved into internal::ServerLifecycle, owned by both server classes. - Location::uri() returns Result<util::Uri>, which needs util::Uri complete (Result embeds AlignedStorage<T>): include arrow/util/uri.h from types.h instead of forward-declaring it. - ToGrpcStatus had no CallbackServerContext overload, needed to attach the Arrow status trailers from a reactor. - Middleware CallCompleted was skipped for unknown methods. - DoPut had no PutResult write-back channel while an upload was in flight. - A PutResult write started while the ending ack was in flight overwrote the write buffer; the ending ack is now deferred until the slot is free. - OnFinish() is application code and may call back into the listener's writer; the transport no longer holds its write lock across it. - gRPC 1.65 is required for the async server only; the async client paths still need 1.40, and ARROW_GRPC_REQUIRED_VERSION returns to 1.30.0. Tests: the async transport tests are split by the transport part each one covers, with the shared fixtures in async_grpc_test_fixtures.h. The synchronous server's suites are unchanged. Known limitations: a blocking ServerAuthHandler is refused (it would run on callback threads) in favour of Handshake/ValidateToken; a handler that abandons its future blocks Shutdown() until a deadline; request arguments are call-scoped.
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