feat: Adding new drafting token with JWT plugin

[victorwoli]

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

Also adding testing files

[snippet-bot]

Here is the summary of changes.

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

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[DinoChiesa]

What is a "drafting token"? is there documentation for this feature beyond "a service extension that can generate and validate JWT tokens"? How does it work? How do the keys get managed? etc etc.