Allow system integrators to specify an update listener

[mxgrey]

As an alternative to (or in combination with) the websocket publishing of updates, this PR allows system integrators to provide their own callback to listen to update messages. System integrators can then do whatever they wish with those messages.

[codecov]

Codecov Report

Merging #198 (867713b) into main (0a14ca8) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
  Coverage   17.85%   17.85%              
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  Files         418      836     +418     
  Lines       38308    76692   +38384     
  Branches    18332    36712   +18380     
==========================================
+ Hits         6838    13697    +6859     
- Misses      24148    48324   +24176     
- Partials     7322    14671    +7349     

Flag	Coverage Δ
tests	17.85% <ø> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
..._rxcpp/RxCpp-4.1.0/Rx/v2/src/rxcpp/rx-observer.hpp
...er/src/rmf_fleet_adapter/agv/FleetUpdateHandle.cpp
...pp/RxCpp-4.1.0/Rx/v2/src/rxcpp/rx-subscription.hpp
...task_ros2/src/rmf_task_ros2/bidding/Auctioneer.cpp
...est/mock_schedule_nodes/missing_query_schedule.cpp
...adapter/src/rmf_fleet_adapter/events/GoToPlace.cpp
..._adapter/src/rmf_fleet_adapter/phases/DoorOpen.cpp
...f_fleet_adapter/agv/internal_RobotUpdateHandle.hpp
...ter/src/rmf_fleet_adapter/agv/EasyTrafficLight.cpp
..._adapter/src/rmf_fleet_adapter/agv/parse_graph.cpp
... and 1244 more

[aaronchongth]

Thanks for adding this API, this will be so useful for upcoming integrations!