logging: Prep for common_log removal

[francislavoie]

See #4148 (comment)

With the upcoming removal of common_log from access logs, there's a few things we should do to smooth things out to offer a path for backwards compatibility.

• Add a common_log time format, which matches the format from the {time.now.common_log} placeholder which is currently used for the common_log field. This will make it easier for users of https://github.com/caddyserver/format-encoder to get the same time format by doing something like this:

  log {
    format formatted <insert common log template here> {
      time_format common_log
    }
  }
  

• Add the http.auth.user.id placeholder's value to the logs. This currently isn't in the access logs, so if we drop common_log, that wouldn't be visible anywhere in the access logs, and therefore not be possible to recover using https://github.com/caddyserver/format-encoder. I went with user_id for the field name but it could just as easily be auth_user_id. 🤷‍♂️

[mholt]

Good first steps. Thanks @francislavoie !

[mholt]

Next steps would probably be to remove the common_log field from the access logs.