[GO] fix missing utils add required supportfiles feature

[NickUfer]

Fixes #13893

This PR adds the new functionality of marking supporting files as required. In some generators (e.g. the GO generator), model and api files depend on the supporting file utils.go. But with settings like --global-property models,modelDocs=false this file is not generated and to find that out that utils.go is missing can be difficult for a user. Required supporting files are always generated, even if they are not specified in the global properties.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.1.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
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@wing328

[wing328]

cc @antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04)

[iuliancristea]

Is there any way in which an outsider can help move this PR further?