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/bench.h

@@ -0,0 +1,19 @@
+#ifndef BENCHMARKS_CPP_BENCH_H_
+#define BENCHMARKS_CPP_BENCH_H_
+
+#include <cstdint>
+
+struct Bench {
+  virtual ~Bench() {}
+
+  inline void Add(int64_t value) { sum += value; }
+
+  virtual uint8_t *Encode(void *buf, int64_t &len) = 0;
+  virtual void *Decode(void *buf, int64_t len) = 0;
+  virtual int64_t Use(void *decoded) = 0;
+  virtual void Dealloc(void *decoded) = 0;
+
+  int64_t sum = 0;
+};
+
+#endif // BENCHMARKS_CPP_BENCH_H_