Add fallback controllers list to the ControllerInfo

[saikishor]

As discussed on 1st April Jazzy meeting, I've created the basic version of adding the fallback controllers list into the controller info and added tests to verify if it is parsing properly or not, we will later continue with the main feature.

This is done as a part of #1468

Better wait for #1502 to be merged first as it might have conflicts with this in the spawner.py (or vice versa)

[christophfroehlich]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 88.42%. Comparing base (0112083) to head (acf764f).

❗ Current head acf764f differs from pull request most recent head 52078e8. Consider uploading reports for the commit 52078e8 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1503   +/-   ##
=======================================
  Coverage   88.42%   88.42%           
=======================================
  Files         101      101           
  Lines        8327     8349   +22     
  Branches      726      728    +2     
=======================================
+ Hits         7363     7383   +20     
  Misses        703      703           
- Partials      261      263    +2     

Flag	Coverage Δ
unittests	88.42% <90.90%> (+<0.01%)	⬆️

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Files	Coverage Δ
controller_manager/test/test_spawner_unspawner.cpp	99.00% <100.00%> (+0.18%)	⬆️
controller_manager/src/controller_manager.cpp	74.85% <66.66%> (-0.05%)	⬇️