Minor change to Helidon version handling allowing snapshot versions

[tjquinno]

The Helidon generators permit users to specify which Helidon version to refer to in the generated code. The code which processes the Helidon version validates the user-specified version against known Helidon releases if possible.

This small enhancement (to the Helidon generators only) allows users to refer to unpublished releases (such as a snapshot release).

The PR includes an additional unit test for this use case.

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:

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  ./bin/utils/export_docs_generators.sh
  

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  Commit all changed files.
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