springboot Add support for customizing Spring Session's cookie serializer

This PR introduces CookieSerializerCustomizer as callback that allows the customization of auto-configured DefaultCookieSerializer bean. This is particularly useful for configuring cookie serializer's capabilities that are not supported by Servlet API and therefore not exposed via server.servlet.session.cookie.* properties, such as SameSite.

The existing RememberMeServicesCookieSerializerCustomizer has also been refactored to leverage the new callback.

Comment From: wilkinsona

Thanks very much, @vpavic.