Build subtarget support for Unity 2021.2+

[danielchipman]

Changes

• Added support for -standaloneBuildSubtarget parameter via customParameters in order to facilitate dedicated server builds. Since Unity introduced the subtarget option in 2021.2, this parameter only takes effect in supported Unity versions.

Documentation PR: game-ci/documentation#389
Discussion: game-ci/documentation#240

Checklist

• Read the contribution guide and accept the
  code of conduct
• Docs (If new inputs or outputs have been added or changes to behavior that should be documented. Please make
  a PR in the documentation repo)
• Readme (updated or not needed)
• Tests (added, updated or not needed)

[github-actions]

[codecov-commenter]

Codecov Report

Merging #532 (77a98c4) into main (3032a4a) will not change coverage.
The diff coverage is n/a.

❗ Current head 77a98c4 differs from pull request most recent head bfc6ff3. Consider uploading reports for the commit bfc6ff3 to get more accurate results

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@@           Coverage Diff            @@
##             main     #532    +/-   ##
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  Coverage   36.97%   36.97%            
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  Files          77       77            
  Lines        3032     3032            
  Branches      586      613    +27     
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  Hits         1121     1121            
+ Misses       1908     1758   -150     
- Partials        3      153   +150     

see 38 files with indirect coverage changes

[davidmfinol]

Thanks!

[danielchipman]

Apologies, did not realize I could run these tests locally on my fork before opening the PR. Seems this API unsurprisingly has gone through some churn and the fallback value I was using isn't available on all editor versions. Swapped it to default (which will resolve to Client) to guarantee compatibility.