Support `kubectl --as`

Currently the proxy deletes all inbound headers used to support impersonation.  This is a major feature used for debugging RBAC and should be supported.  In order to support:

1. When there are impersonation headers, do a `SubjectAccessReview` call to validate the user is able to impersonate the inbound headers
2. Log that the impersonation has happened using the simple log framework from #2 
