Implement PoC for Thread-Safety annotation

Clang doc here: https://clang.llvm.org/docs/ThreadSafetyAnalysis.html

Candidate class for the PoC is https://github.com/ros2/rmw_fastrtps/blob/705a0c07ca9f3708028660c41de6eadfad16e9f5/rmw_fastrtps_shared_cpp/include/rmw_fastrtps_shared_cpp/topic_cache.hpp

Owner: @emersonknapp / @ros2/aws-robotics

Exit Criteria

• Add Thread-Safety annotations macro to e.g. rcutils (see mutex.h in https://clang.llvm.org/docs/ThreadSafetyAnalysis.html) - which repo should be cleared with OSRF first
• Modify rmw_fastrtps* packages to build with -Wthread-safety by default when Clang is chosen.
• Modify topic_cache.hpp to use thread-safety annotations.

Progress Tracking

• Discourse: Adding clang thread safety analysis for ROS2 core packages
• Add thread annotation macros rcpputils#2
• Add rcpputils to ros2.repos #661

Connects to #662