* Rework flatbuffers + gRPC integration
- Introduce `flatbuffers::grpc::Message<T>`, a `grpc_slice`-backed
message buffer that handles refcounting and allows flatbuffers to
transfer ownership to gRPC efficiently. This replaces
`flatbuffers::BufferRef<T>`, which required a copy call and was also
unsafe w.r.t. buffer lifetime.
- Introduce `flatbuffers::grpc::MessageBuilder`, a gRPC-specific builder
that forces a `grpc_slice`-backed allocator and also adds some helpful
`Message<T>`-related methods.
- Update serializers accordingly (now zero-copy between flatbuffers and
gRPC).
* gRPC: verify messages by default, but allow user to override
* gRPC: fix some formatting issues
* Disable verification by default, but add helper method
* Make FlatBufferBuilder fields protected + remove vec accessor
* Use bool add_ref parameter to toggle refcount incr
* Remove unnecessary inline specifiers
* Fix formatting
* Use auto
* Remove empty lines
* Use grpc_slice helper macros
* Simplify reset code
* Disable Message copy ctor and assignment by default
* Remove unused member
* Enable gRPC verification by default
* Use auto
* Bake in message verification (remove template specialization)
* Add RoundUp func
* Consolidate gRPC message copy flag
* Make vector_downward allocations fully lazy
* Test message verification failure code/message
* Add grpctest verification test comments
* Simplify reallocate implementation
* Make initial_size a size_t
* Use ternary op for growth_policy
* Use truthiness rather than dont explicit nullptr check
* Indent preprocessor directives
* Remove grpc message copy/assignment
* Fix a few bugs
* Add gRPC example
* Add basic gRPC docs
* Use doxygen EXAMPLE_PATH + @include
* Reference example fbs in grpc docs
* Move gRPC examples into grpc/samples
* Fix pointer/reference formatting
* Use std::function rather than templated callback func
* Create fresh message builder for each request
* Use Clear() in Reset() impl
* Use FLATBUFFERS_CONSTEXPR
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