Refactor build user vars plugin: Remove unnecessary utility class, relocated constants to a more generic context

[fabiodcasilva]

Refactor build user vars plugin:

• Remove unnecessary utility class.
• Relocated constants to a more generic context class.
• Add missing tests to UsernameUtils

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[fabiodcasilva]

@basil can I kindly ask you for a review?

[fabiodcasilva]

@oleg-nenashev can you please review this PR?

[basil]

Sorry for the late reply. This looks great!

[fabiodcasilva]

Sorry for the late reply. This looks great!

thanks!