Add in support for both the PMR and custom allocator tutorials.

[clalancette]

This is mostly because libc++ 14 (in Ubuntu 22.04) only understands the old version. So we choose the appropriate file based on the compiler in use.

Note that this partially undoes #653 , at least by adding in both versions, and then selecting at compile-time. This should fix our failing Clang builds at https://ci.ros2.org/view/nightly/job/nightly_linux_clang_libcxx/

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows

[clalancette]

Oh, and clang:

• Clang