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.

You are about to add 1 region tag.

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

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment