refactor(conf_loader): separate constants from conf_loader core

[chronolaw]

Summary

conf_loader.init has too many lines and is too complex, it is hard to understand and maintain,
I try to move out some code to simplify it.

KAG-3128

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

Full changelog

• a new module conf_loader.constants
• cipher_suites => CIPHER_SUITES
• typ_checks => TYP_CHECKS
• _NOP_TOSTRING_MT
• use CONF_SENSITIVE_PLACEHOLDER to replace "******"

Issue reference

Fix #[issue number]

[gszr]

Nice refactor; left a minor readability comment. Otherwise, LGTM.

[tzssangglass]

LGTM