Currently the 'user-id' field of the authorize logs is empty for
requests authenticated via a service account, as there is no associated
User object. Instead, populate this log field directly from the the
sessionOrServiceAccount value, to handle both types of user.
* config: add customization options for logging
* config: validate log fields
* proxy: add support for logging http request headers
* log subset of headers
* add support for logging the http query
* fix test name
* use strings.Cut, add unit tests
