feat(cas-auth): support logged-in user header

[nufissime]

Description

The cas-auth plugin is great, but does not currently support retrieving the logged-in user from upstreams, making it really hard to do any kind of access control appart from "a user is logged-in".

This PR adds the set_user_header and user_header attributes that enables setting the logged-in user in a header, and allows for the header's name customization.

Some users have already asked for it.

As far as backward compatibility is concerned: The header is set by default, so one might experience issues if setting the header before reaching apisix... which would make little sense considering apisix handles the authentication.

Fixes #9524.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)