Add unique_lock implementation with clang thread safety annotations

[emersonknapp]

Can be used to replace rosbag2 implementation https://github.com/ros2/rosbag2/blob/rolling/rosbag2_cpp/src/rosbag2_cpp/clocks/time_controller_clock.cpp#L31
and false positive errors in RMW builds like https://ci.ros2.org/view/nightly/job/nightly_linux_clang_libcxx/1628/clang/new/source.cf52365c-c9dc-4c3e-8639-55ffa17e1b64/#115

[emersonknapp]

Probably needs a test

[fujitatomoya]

having this in rcpputil makes sense, test would be ideal.

[clalancette]

And just for verification, it looks like using this instead of std::unique_lock in rmw_fastrtps makes a difference: ros2/rmw_fastrtps#712 (comment) (there are about 140 fewer violations than on e.g. https://ci.ros2.org/view/nightly/job/nightly_linux_clang_libcxx/1629/)

[clalancette]

I've left a possible bug inline.

As far as a test goes, I'm actually not entirely sure what we would test here. More-or-less it would be testing whether std::unique_lock works. Any thoughts from @emersonknapp or @fujitatomoya on what kind of test you think we should do?

[fujitatomoya]

@clalancette that is actually good question. i was thinking that we can simulate https://ci.ros2.org/view/nightly/job/nightly_linux_clang_libcxx/1628/clang/new/source.cf52365c-c9dc-4c3e-8639-55ffa17e1b64/#115 but it actually is the test for RCPPUTILS_TSA_SCOPED_CAPABILITY.

we could test rcpputils::unique_lock works, but i dont think that is even necessary either.

[clalancette]

@emersonknapp When you get a chance, can you rebase this to fix the DCO bot?

[emersonknapp]

Added a simple compile-test that we can use multiple types of mutex.

[emersonknapp]

Pulls: #180
Gist: https://gist.githubusercontent.com/emersonknapp/c4a008cb46d56f9548cffbcbaefac5a1/raw/22101fab9d965b83665b049428add3a1a64e682a/ros2.repos
BUILD args: --packages-above-and-dependencies rcpputils
TEST args: --packages-above rcpputils
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/12608

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