feature: PKCE challange

[Hero9909]

A clear and concise description of what you want the system to do.

Within the OAuth2 authorization flow it would be great to support the PKCE challange, as it improves the security.
I was noticing this because Kanidm enforces this by default.

What are the Caddyfile directives that need to be added.

Add Caddyfile directive:

{
  security {
    oauth identity provider generic {
      use_pkce true|false
    }
  }
}