feat(plugin/remote-auth): plugin to auth requests via remote server

[james-zinger]

Summary

Adding a new plugin to handle authentication requests.

Requests are made via http/https to a remote where most headers are
configurable with defaults.

The core requirements are the url for the authentication server and the
JWT public key to verify key integrity.

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

None

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