Pass down SignedUserName down to AccessLogger context (#16605)

Pass down SignedUserName down to AccessLogger context (#16605)

* Pass down SignedUserName down to AccessLogger context

Unfortunately when the AccessLogger was moved back before the contexters the
SignedUserName reporting was lost. This is due to Request.WithContext leading to a
shallow copy of the Request and the modules/context/Context being within that request.

This PR adds a new context variable of a string pointer which is set and handled
in the contexters.

Fix #16600

Signed-off-by: Andrew Thornton <art27@cantab.net>
* handle nil ptr issue

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>