enhance generic throttle

[fmessmer]

follow up of https://github.com/ipa320/msh/issues/727

generic throttle cannot deal with the following properly:

• local/global_costmap: the rviz display uses a display that visualizes both costmap and costmap_updates topic
• image_transport: just subscribing to a compressed image, throttling it and publishing it as topic_throttled does not work in rviz either...i.e. it cannot have the suffix _throttled...this needs to be investigated in more detail

@fmessmer FYI

[AravindaDP]

One more feature I like to have is a parameter for repeating last_message in the given rate when no new messages are received. This would allow a topic to have a uniform rate even when source node publishing it in sporadic intervals.

I'm willing to submit this enhancement in a PR if you think it's worthwhile feature

[fmessmer]

@AravindaDP
I don't see an immediate usecase for this - but I'm willing to review/merge a PR that supports your feature as an opt-in

[fmessmer]

not planned