http2: make Http2Settings constructors delegate

This commit extracts the common code in the existing Http2Settings
constructors into a private constructor, allowing the existing ones to
delegate to the private constructor it and avoid code duplication.

PR-URL: https://github.com/nodejs/node/pull/23326


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>