Message Count API Creation

[CursedRock17]

This PR is meant in preparation for issue #1109 in the rcl repository by creating a rmw API for counting messages a Subscription is ready to receive.

[fujitatomoya]

@CursedRock17 thanks for creating PR.

IMO, this requires RMW change, i would suggest that before moving forward, probably it would be nice to have discussion on basic design describes how we could implement this underlying rmw implementation, including client interfaces? maybe REP required, i am not sure... @clalancette @wjwwood WDYT?

[clalancette]

IMO, this requires RMW change, i would suggest that before moving forward, probably it would be nice to have discussion on basic design describes how we could implement this underlying rmw implementation, including client interfaces? maybe REP required, i am not sure... @clalancette @wjwwood WDYT?

Yeah, agreed. I'd like to see draft PRs, or at least documentation, on how we would implement this for rmw_fastrtps, rmw_cyclonedds, rmw_connextdds, and rmw_zenoh.