This change allows for the caller of the `saml/prepare` API to pass
a `relay_state` parameter that will then be part of the redirect
URL in the response as the `RelayState` query parameter.
The SAML IdP is required to reflect back the value of that relay
state when sending a SAML Response. The caller of the APIs can
then, when receiving the SAML Response, read and consume the value
as it see fit.