feat: Adding new drafting token with JWT plugin (Go Version)

[victorwoli]

Adding new drafting token with JWT plugin. This will be a Google Cloud service extension that can generate and validate JWT tokens.

[snippet-bot]

Here is the summary of changes.

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• plugins/samples/drafting_jwt_token/plugin.go:15, tag serviceextensions_plugin_drafting_jwt_token

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[mpwarres]

@leonm1 when you have a chance can you review this? Thanks!

[martijneken]

Also address any general comments from the C++ plugin (e.g. testing)