Add plugin key-token.

[BinJu]

The plugin takes the given key from a configured header name of the incoming traffic, then accesses the configured authentication service for a (JWT) token. The token is cached as the secified TTL and inserted to the request header to access the target service.

TODOs:

• Add more test coverage for both unit and integration.
• Understand where to put the doc for this plugin.
• Need more detailed granular control of accessing authN/authZ server. For example the timeout, return code handling.
• Need more discussion on the high level design of this plugin. Does the actual use case exist? And how to make it more reliable and flexible.

Summary

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #[issue number]

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