[csharp] Changed enums to start from 0

[devhl-labs]

I have some enumerations with "Unknown" as a valid option. The generated code was giving this enum a value of 1. This is a bit unusual, I would expect this value to be 0. To solve this, I've added logic to allow the enum to be zero based if "Unknown" (case insensitive) is the first option, or if the enum is marked as x-zero-based-enum, or if the user adds zeroBasedEnum=true to the additional properties.

This fixes the issue with a fallback, users can opt out. Also, if the enumeration does not have "Unknown" as the first value, then the enum will still start at 1, giving developers the ability to add unknown as the first option later without messing up the enum values.

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.
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  ./bin/utils/ensure-up-to-date
  

  Commit all changed files.
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[devhl-labs]

@mandrean @frankyjuang @shibayan @Blackclaws @lucamazzanti

[devhl-labs]

Failure is unrelated.