Added Google benchmarks (and gtests) (#6920)

* Added Google benchmarks (and gtests)

* Default building benchmarks to OFF as it requires c++11

* Separate benchmark CMakeLists.txt to its own file

* Move output directory to target just flatbenchmark

benchmarks/cpp/benchmark_main.cpp

@@ -0,0 +1,61 @@
+#include <benchmark/benchmark.h>
+#include <gtest/gtest.h>
+
+#include "benchmarks/cpp/bench.h"
+#include "benchmarks/cpp/flatbuffers/fb_bench.h"
+
+static void BM_Flatbuffers_Encode(benchmark::State &state) {
+  const int64_t kBufferLength = 1024;
+  uint8_t buffer[kBufferLength];
+
+  int64_t length;
+
+  StaticAllocator allocator(&buffer[0]);
+  std::unique_ptr<Bench> bench = NewFlatBuffersBench(kBufferLength, &allocator);
+
+  for (auto _ : state) {
+    bench->Encode(buffer, length);
+    benchmark::DoNotOptimize(length);
+  }
+}
+BENCHMARK(BM_Flatbuffers_Encode);
+
+static void BM_Flatbuffers_Decode(benchmark::State &state) {
+  const int64_t kBufferLength = 1024;
+  uint8_t buffer[kBufferLength];
+
+  int64_t length;
+
+  StaticAllocator allocator(&buffer[0]);
+  std::unique_ptr<Bench> bench = NewFlatBuffersBench(kBufferLength, &allocator);
+
+  uint8_t* encoded = bench->Encode(buffer, length);
+  
+  for (auto _ : state) {
+    void* decoded = bench->Decode(encoded, length);
+    benchmark::DoNotOptimize(decoded);
+  }
+}
+BENCHMARK(BM_Flatbuffers_Decode);
+
+static void BM_Flatbuffers_Use(benchmark::State &state) {
+  const int64_t kBufferLength = 1024;
+  uint8_t buffer[kBufferLength];
+
+  int64_t length;
+
+  StaticAllocator allocator(&buffer[0]);
+  std::unique_ptr<Bench> bench = NewFlatBuffersBench(kBufferLength, &allocator);
+
+  uint8_t* encoded = bench->Encode(buffer, length);
+  void* decoded = bench->Decode(encoded, length);
+
+  int64_t sum = 0;
+
+  for (auto _ : state) {
+    sum = bench->Use(decoded);
+  }
+
+  EXPECT_EQ(sum , 218812692406581874);
+}
+BENCHMARK(BM_Flatbuffers_Use);