feat(custom-auth): Add a plugin to do authentication with remote server

[iykon]

Summary

feat(custom-auth): Add a plugin to do authentication with remote server

• The plugin requires two mandatary fields, remote auth server and request header name. On receiving a request, the plugin will contact remove auth server with provided request header.
• The plugin caches the response, with configurable ttl.
• The plugin optionally gets a configurable header from remove server response and includes as the header to upstream service.

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

NA

[CLAassistant]

All committers have signed the CLA.