Adding rmw_request_id_t allocator

[CursedRock17]

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Feature request

Should rmw_request_id_t's allocator be used in the create_request_header method

Feature description

Added in this commit, there is a TODO within the method create_request_header that wants to utilize rmw_request_id_t with a custom allocator of some sort.

Implementation considerations

This TODO is roughly 8.5 years old, a lot has changed since then especially with rmw, so if this isn't necessary I can just open a PR that removes the TODO. Otherwise, since rmw_request_id_t doesn't have an allocator inside of the class, should this method be using some from of allocator in rclc since it is a C type, in order to efficiently create new request headers?

[fujitatomoya]

@CursedRock17 i am not sure what needs to be done here.

@wjwwood can you share your idea or example?

[clalancette]

I'm not 100% sure, but my guess is that the idea is to honor the custom rcutils allocator if the user has set it. So this is probably still valid? Unclear to me. I guess we can wait a little longer for @wjwwood to respond.