0.4: allow passing login username through request headers for proxy auth.

[pirogoeth]

Around access.lua#L429, we should be able to add an ngx.req.set_header call to set a header (X-LSSO-Session -- but configurable?) so that the upstream service can use that as an auth method.

This is particularly useful for web applications supporting proxy auth, such as Grafana. This is a good step toward making lsso more powerful.