Rationale for this change

Currently the Serialize/Deserialize APIs are gRPC dependent. This means that any code that needs to encode or decode Flight data must depend on gRPC C++ internals. After some discussions around trying to build a PoC using gRPC's generic API with gRPC's BidiReactor we discussed that these primitives should be made gRPC agnostic.

What changes are included in this PR?

• Move the serialization/deserialization logic from cpp/src/arrow/flight/transport/grpc/serialization_internal.{h,cc} to cpp/src/arrow/flight/serialization_internal.cc
• Create new arrow::Result<arrow::BufferVector> SerializePayloadToBuffers(const FlightPayload& msg) gRPC agnostic function.
• Create new arrow::Result<arrow::flight::internal::FlightData> DeserializeFlightData(const std::shared_ptr<arrow::Buffer>& buffer) gRPC agnostic function.
• Keep the existing serialize/deserialize functions for gRPC as simple wrappers on top of the new serialization functions to implement the grpc::ByteBuffer and grpc::Slice details.
• Add utility arrow::Result<BufferVector> SerializeToBuffers() const; to FlightPayload struct.
• Add roundtrip tests to serialize/deserialize

Are these changes tested?

Yes, both by existing tests and new tests.

Are there any user-facing changes?

No

• GitHub Issue: [C++][FlightRPC] Decouple Flight Serialize/Deserialize from gRPC transport #49548