Spring Expose HttpHandler Decoration as a bean

Follows the development that has been done for https://github.com/spring-projects/spring-framework/issues/25633.

Having the httpHandlerDecorator as a bean allows easier integration with Spring Boot, see HttpHandlerAutoConfiguration.

I wasn't sure what version to put in @since Javadoc tags.

Let me know what you think.

Comment From: pivotal-issuemaster

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Comment From: rstoyanchev

Something like this would be fine.

Comment From: rstoyanchev

A couple of things I'll probably adjust. 1) we'd want to allow multiples of these, and 2) it should be named something other than Decorator for consistency with other such types in our web code which normally are implementations of the target type (HttpHandlerDecoratorFactory perhaps).

Comment From: sp00m

@rstoyanchev, thanks for the review, updated as proposed, let me know.