Add extra joints parameter at joint state broadcaster

[cesc-folch]

Adding a feature that existed in ros1 and was not migrated yet.

[codecov-commenter]

Codecov Report

Merging #179 (2c117ef) into master (189a2f9) will decrease coverage by 0.34%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
- Coverage   35.28%   34.93%   -0.35%     
==========================================
  Files          26       26              
  Lines        2256     2290      +34     
  Branches     1462     1493      +31     
==========================================
+ Hits          796      800       +4     
  Misses        132      132              
- Partials     1328     1358      +30     

Flag	Coverage Δ
unittests	34.93% <ø> (-0.35%)	⬇️

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

Impacted Files	Coverage Δ
..._controllers/src/joint_group_effort_controller.cpp
...ontrollers/src/joint_group_position_controller.cpp
...include/joint_trajectory_controller/trajectory.hpp
...mand_controller/src/forward_command_controller.cpp
...int_trajectory_controller/test/test_trajectory.cpp
...ler/test/test_load_joint_trajectory_controller.cpp
...ollers/test/test_joint_group_effort_controller.cpp
..._state_broadcaster/src/joint_state_broadcaster.cpp
...lers/test/test_joint_group_velocity_controller.cpp
...s/test/test_load_joint_group_effort_controller.cpp
... and 42 more

[bmagyar]

The PR looks good, good attention to detail with adding tests on first submission! ;)
I'd like to ask one thing however: please add a section on parameters with a very brief explanation as to what is expected when this parameter is specified to joint_state_broadcaster/doc/userdoc.rst

[jordan-palacios]

LGTM!