SG-17191 Added the ability to flag certain includes as optional

[eshokrgozar]

Allows the user to flag an include as not required (optional). If an optional include resolves to a missing file, a warning is logged instead of raising an Exception.

Here's the new supported syntax (it still supports the old way too):

includes:
- {path: ./path/to/the_file.yml, required: false}

[codecov]

Codecov Report

Merging #765 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #765      +/-   ##
==========================================
+ Coverage   78.75%   78.85%   +0.10%     
==========================================
  Files         177      177              
  Lines       18797    18789       -8     
==========================================
+ Hits        14803    14817      +14     
+ Misses       3994     3972      -22     

Impacted Files	Coverage Δ
python/tank/platform/environment_includes.py	92.22% <100.00%> (+13.92%)	⬆️
python/tank/template_includes.py	66.33% <100.00%> (-2.69%)	⬇️
python/tank/util/includes.py	100.00% <100.00%> (ø)
python/tank/util/filesystem.py	86.17% <0.00%> (-0.93%)	⬇️
python/tank/platform/engine.py	62.73% <0.00%> (-0.07%)	⬇️
python/tank/util/unicode.py	100.00% <0.00%> (ø)
python/tank/platform/environment.py	86.65% <0.00%> (+0.56%)	⬆️
python/tank/pipelineconfig.py	85.84% <0.00%> (+0.60%)	⬆️

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