Make the stubborn negotiator's strategy more flexible

[mxgrey]

The StubbornNegotiator negotiation strategy may be too rigid to be a good fit for some tricky circumstances. This PR provides more knobs to tailor the strategy of a StubbornNegotiator to better suit various circumstances.

These changes are particularly important if multiple read-only traffic participants need to co-exist in the same space. The new stubborn negotiation options allows conflicts between read-only participants to be resolved outside of RMF (e.g. by human operators) while still resolving any conflicts between the read-only and autonomous participants.

[codecov]

Codecov Report

Merging #40 (c0090c6) into main (b2ebd93) will decrease coverage by 0.02%.
The diff coverage is 33.04%.

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   32.66%   32.64%   -0.03%     
==========================================
  Files         159      159              
  Lines       15655    15729      +74     
  Branches    10449    10489      +40     
==========================================
+ Hits         5114     5134      +20     
- Misses       1702     1736      +34     
- Partials     8839     8859      +20     

Flag	Coverage Δ
tests	32.64% <33.04%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
...traffic/include/rmf_traffic/agv/RouteValidator.hpp	40.00% <ø> (+15.00%)	⬆️
...nclude/rmf_traffic/schedule/StubbornNegotiator.hpp	0.00% <ø> (ø)
...ic/src/rmf_traffic/schedule/StubbornNegotiator.cpp	20.27% <18.84%> (-29.73%)	⬇️
rmf_traffic/src/rmf_traffic/agv/RouteValidator.cpp	47.11% <54.34%> (+2.98%)	⬆️
...f_traffic/src/rmf_traffic/schedule/Negotiation.cpp	54.86% <0.00%> (ø)