Add support for model config dependencies

[JShep1]

Downloads the dependencies listed in a model.config file, depends on gazebosim/gz-msgs#91

Signed-off-by: John Shepherd [email protected]

[codecov]

Codecov Report

Merging #124 into jshep1/download_dependencies will decrease coverage by 0.09%.
The diff coverage is 45.45%.

@@                       Coverage Diff                        @@
##           jshep1/download_dependencies     #124      +/-   ##
================================================================
- Coverage                         76.92%   76.82%   -0.10%     
================================================================
  Files                                19       19              
  Lines                              2548     2559      +11     
================================================================
+ Hits                               1960     1966       +6     
- Misses                              588      593       +5     

Impacted Files	Coverage Δ
src/FuelClient.cc	67.09% <45.45%> (-0.23%)	⬇️

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