Allow fleet adapters to manually activate stubborn negotiation

[mxgrey]

This PR adds an unstable feature to allow system integrators to manually activate stubborn negotiation. It also provides a simple API for overriding a robot's schedule to say that it is holding at a position.

These are experimental features so they are all in the Unstable API for now.

[codecov]

Codecov Report

Merging #196 (f4d82fe) into main (e9743b9) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
- Coverage   17.85%   17.84%   -0.02%     
==========================================
  Files         418      836     +418     
  Lines       38246    76616   +38370     
  Branches    18302    36664   +18362     
==========================================
+ Hits         6830    13673    +6843     
- Misses      24096    48290   +24194     
- Partials     7320    14653    +7333     

Flag	Coverage Δ
tests	17.84% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...adapter/src/rmf_fleet_adapter/phases/DockRobot.cpp
...8gyq/rmf_ros2/rmf_fleet_adapter/test/test_Task.cpp
...ffic_ros2/src/rmf_traffic_ros2/schedule/Writer.cpp
...mf_ros2/rmf_task_ros2/src/dispatcher_node/main.cpp
...dapter/src/read_only_blockade/FleetAdapterNode.hpp
...rxcpp/RxCpp-4.1.0/Rx/v2/src/rxcpp/rx-scheduler.hpp
.../rmf_traffic_ros2/test/unit/test_convert_Graph.cpp
...os2/rmf_fleet_adapter/src/task_aggregator/main.cpp
...affic_ros2/src/rmf_traffic_ros2/geometry/Shape.cpp
...et_adapter/src/rmf_fleet_adapter/tasks/Compose.cpp
... and 1244 more