[service introspection] Guard `#include <cstddef>` when being included from a `c` source

[ihasdapie]

This PR is a prototype for the service introspection REP (https://github.com/ros-infrastructure/rep/pull/360) implementing the proposed single serialized message approach (https://github.com/ros-infrastructure/rep/pull/360#discussion_r902987531).

This PR wraps #include <cstddef> in rosidl_typesupport_c/include/rosidl_typesupport_c/type_support_map.h so that rcl can access type_support_map_t

See: https://github.com/ros2/rcl/pull/990

For instructions on how to build and run this see the meta-ticket https://github.com/ros2/ros2/issues/1285.

Resolves https://github.com/ros2/ros2/issues/1285

[sloretz]

@ihasdapie I see this is marked as a draft, but it looks fine as is. Mind if I run CI and merge it?

[ihasdapie]

@sloretz Np, I've just made a CI job for it

• Linux
• Linux-aarch64
• Windows

[ihasdapie]

Ignore the previous comment, needed to make a new branch for CI to avoid pulling in my changes in other repos...

• Linux
• Linux-aarch64
• Windows

[jacobperron]

@ihasdapie Be careful to use "Resolves #X" in the ticket description only when it should close the connected ticket. Merging this closed the meta-ticket tracking the service introspection work.

[ihasdapie]

@jacobperron Got it. I had originally planned on merging them at the same time but in retrospect that might be a bit too hopeful.