[Oauth]Add PKCE option for github(GF_AUTH_GITHUB_USE_PKCE)

Support PKCE in GitHub OAuth. ### Why is this needed - follow-up #68073 - relates #64746 GitHub did not support PKCE before, but now supportd PKCE. https://github.blog/changelog/2025-07-14-pkce-support-for-oauth-and-github-app-authentication/ ### What would you like to be added [Add a brief description of what the feature or enhancement should do.] Add `use_pkce` to defaults.ini. **Who is this feature for?** Users for using Github Oauth