Fix logout redirects to custom URL schemas

[liujed]

Post-logout redirect URLs with custom URL schemas were incorrectly being treated as CAS-relative paths. This resulted in, for example, a redirect intended for custom://foo being sent instead to
https://localhost:8443/cas/custom://foo.

To address this, the post-logout redirect is now sent through CAS's OAuth2 callbackAuthorize endpoint, which handles custom URL schemas correctly.

Unfortunately, Puppeteer doesn't interact well with custom URL schemes:

• When an unregistered scheme like custom:// is used, Chromium requires manual intervention to pass the URL onto Linux's xdg-open, which Puppeteer is unable to observe.
• When a well-known scheme like mailto: is used, this leaks out into the OS's mailto handler, which Puppeteer is also unable to observe.
• Having Puppeteer intercept an unregistered scheme isn't an option, according to how can i Intercept custom URL schemes request? puppeteer/puppeteer#9683.

This makes it challenging to write a comprehensive functional test. Instead, this PR has a negative test that checks that the custom URL scheme isn't treated as a relative path.

• Brief description of changes applied
• Test cases for all modified changes, where applicable
• Any possible limitations, side effects, etc

[mmoayyed]

Thank you for submitting this patch.

1. This change ideally needs to be supported by Spring Webflow; ATM, external redirects are only supported by Webflow when the scheme is either http, https or some form of context/servlet relative URL. When and if spring webflow supports custom schemes, then this should work OOTB. Please contact Spring Webflow and raise this case with them before taking action here.
2. Your puppeteer test needs to test the fact that redirect DID in fact happen; you do not need to intercept but only make sure the browser attempted to redirect to the URL given.
3. Please do not copy puppeteer tests wholesale into a whole new test scenario without cleanup and modifications to only test the bit you care about. Or, modify an existing test to add the variation you need.

[liujed]

Thanks, Misagh. Patch submitted upstream as spring-projects/spring-webflow#1841.