feat: Replace default Go user-agent with oauth2-proxy and version (#2570)

* feat: Replace default Go user-agent with oauth2-proxy and version

* Add to CHANGELOG

* Make userAgentTransport configurable and composable

* Use correct naming convention for DefaultHTTPClient

* Move version to own package and use named arguments

* Update version path in Makefile

* Fix import path in Makefile

* Change importpath in dist.sh

* Minor style issues