[Cookbook][Security] Added doc for x509 pre authenticated listener

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

This can be merged in 2.3. Regards.
Hmm, and I'm thinking that we should move this down to the end of the entry, and mix it with the conversation about `SSL_CLIENT_S_DN_Email` and mention the `your_user_provider` key specifically. I think people will jump straight down to the code block and ask: "What is this `your_user_provider`?". I want to tell them that `your_user_provider` is a user provider, here are some links, and that the `SSL_CLIENT_S_DN_Email` will be passed in as the username to the user provider.

And what is the use-cause for needing `SSL_CLIENT_S_DN`? I'm not familiar with X509. I'm asking because usually, once a user is authenticated (which is taken care of by the core 509 classes), you don't care about the Token class. And I think that the credentials on the token are even cleared after authentication (unless `security.erase_credentials` is set to `false`).

Thanks!