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

.gitignore

@@ -46,6 +46,8 @@ project.properties
 proguard-project.txt
 linklint_results
 Makefile
+flatbenchmark
+flatbenchmark.exe
 flatc
 flatc.exe
 flathash
@@ -143,3 +145,5 @@ flatbuffers.pc
 **/FlatBuffers.Test.Swift.xcodeproj
 **/html/**
 **/latex/**
+# https://cmake.org/cmake/help/latest/module/FetchContent.html#variable:FETCHCONTENT_BASE_DIR
+_deps/