Make the cookie secure flag configurable in CookieServerCsrfTokenRepository

[ThomasVitale]

Expected Behavior

The CookieCsrfTokenRepository for Spring MVC applications supports configuring the secure flag for the XSRF-TOKEN cookie. The CookieServerCsrfTokenRepository for Spring WebFlux applications should provide the same option.

Current Behavior

The CookieServerCsrfTokenRepository for Spring WebFlux applications doesn't allow configuring the secure flag for the XSRF-TOKEN cookie.

Context

The secure flag is currently set to true when the TLS termination is on the application itself. If it's on a firewall or proxy, then it's always false.

Reference to the same change done for servlet-based applications: #8749

Suggested solution: #9679 (PR)