flatbuffers support(3/3): Add flatbuffers protocol support for brpc #2584

Job	Run time
clang-unittest-asan	58m 35s
clang-compile-with-bazel	9m 10s
gcc-compile-with-boringssl	8m 52s
clang-compile-with-boringssl	11m 47s
bazel-bvar-unittest	19m 0s
gcc-compile-with-make-protobuf	4m 57s
clang-compile-with-make-protobuf	4m 51s
clang-unittest	5m 48s
compile-with-make	8m 11s
gcc-compile-with-bazel	7m 30s
compile-with-cmake	4m 19s
clang-compile-with-bazel-all-options	11m 37s
gcc-compile-with-bazel-all-options	8m 22s
	2h 42m 59s

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