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/flatbuffers/fb_bench.h

@@ -0,0 +1,23 @@
+#ifndef BENCHMARKS_CPP_FLATBUFFERS_FB_BENCH_H_
+#define BENCHMARKS_CPP_FLATBUFFERS_FB_BENCH_H_
+
+#include <cstdint>
+#include <memory>
+
+#include "benchmarks/cpp/bench.h"
+#include "include/flatbuffers/flatbuffers.h"
+
+struct StaticAllocator : public flatbuffers::Allocator {
+  explicit StaticAllocator(uint8_t *buffer) : buffer_(buffer) {}
+
+  uint8_t *allocate(size_t) override { return buffer_; }
+
+  void deallocate(uint8_t *, size_t) override {}
+
+  uint8_t *buffer_;
+};
+
+std::unique_ptr<Bench> NewFlatBuffersBench(
+    int64_t initial_size = 1024, flatbuffers::Allocator *allocator = nullptr);
+
+#endif  // BENCHMARKS_CPP_FLATBUFFERS_FB_BENCH_H_