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

[chronolaw]

Summary

Same like #11569

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

• [Implement ...]

Issue reference

Fix #[issue number]

[chronolaw]

Plugin: oauth2 [#postgres] access OAuth2 Access Token succeeds when code challenge contains + or / characters
spec/03-plugins/25-oauth2/03-access_spec.lua:2507: Invalid response status code.
Status expected:
(number) 200
Status received:
(number) [400](https://github.com/Kong/kong/actions/runs/6939962713/job/18878143542?pr=12069#step:16:401)
Body:
(string) '{"error":"invalid_grant","error_description":"Invalid code"}'
200

But I have no time to solve it.

[chronolaw]

Still got ci test failure.