Remove class WP-Auth0 and move methods to functions

[joshcanhelp]

Changes

• Remove class WP_Auth0
• Move WP_Auth0::ready() to wp_auth0_is_ready()
• Move WP_Auth0::get_tenant_region() to wp_auth0_get_tenant_region()
• Move WP_Auth0::get_tenant() to wp_auth0_get_tenant()
• Move WP_Auth0::render_back_to_auth0() to WP_Auth0_Lock::render_back_to_lock()

Note to reviewers: This PR is piecing the WP_Auth0 class into functions. I would recommend going commit by commit for this review. The diff for WP_Auth0.php did not come out well so if you're looking at the changed file, lines ~40 - 285 are all new.

Testing

• This change adds unit test coverage
• This change has been tested on WP5.3

Checklist

• All existing and new tests complete without errors
• All code quality tools/guidelines in the Contribution guide have been run/followed
• All active GitHub CI checks have passed