refactor(plugins/jwt): use encode_base64url in resty.core

[chronolaw]

Summary

lua-resty-core provides two functions base64.encode_base64url() and base64.decode_base64url(),
Which have the same effect as our Lua functions.

Checklist

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

Full changelog

• use base64.encode_base64url and base64.decode_base64url
• optimize table insert

Issue reference

Fix #[issue number]