Enable OIDC debug

[danielnginx]

Is your feature request related to a problem? Please describe.

Allow NGINX Ingress Controller users to enable the OIDC debug flag available in this Pull Request: https://github.com/nginxinc/nginx-openid-connect/pull/114/files#diff-56939fdacf34acf014e8b512c3fab0b7bd601411e001469c552a8844c9a8da3aR5

#set $oidc_debug "";

[haywoodsh]

Add mention to the PR: nginxinc/nginx-openid-connect#114