Spring Allow for AbstractUrlHandlerMapping to add/remove handlers at runtime

Closes #26798

Made void registerHandler(String urlPath, Object handler) throws BeansException, IllegalStateException public and introduced a public unregisterHalder(String urlPath).

Makes possible to add and remove handlers at runtime.

Comment From: pivotal-cla

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Comment From: pivotal-cla

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Comment From: snicoll

@awmeinema thanks for making your first contribution to Spring Framework. I've polished things up a bit and added tests in ad0c488