[Go] Implement GetActualInstanceValue method

[Sander0542]

When you are using a oneOf type inside a path parameter, the value in the URL is set to the whole parameter object instead of the actual value.

I implemented a GetActualInstanceValue which returns the value of the object and a type assertion to check for the method. By doing it like this the path parameters will support oneOf parameters in the Go client.

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 || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  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*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04) @lwj5 (2023/04)

Closes #14028

[wing328]

thanks for the PR

can you please PM me when you've time to further discuss this?

[Sander0542]

can you please PM me when you've time to further discuss this?

@wing328 I have time to discuss this in the coming 6 hours. Does that suit you?

[Sander0542]

@wing328 when do you have time to dicuss the changes?

[wing328]

what about coming week, say Tuesday? my timezone is +0800 (Hong Kong)