Support for passing through URL query parameters from /oauth2/start to the ID provider's login URL.

You must explicitly configure oauth2-proxy (alpha config only) with which parameters are allowed to pass through, and optionally provide an allow-list of valid values and/or regular expressions for each one.  Note that this mechanism subsumes the functionality of the "prompt", "approval_prompt" and "acr_values" legacy configuration options, which must be converted to the equivalent YAML when running in alpha config mode.