Improve support for WebFlux RouterFunction

[neiser]

Open points:

• The RouterFunctionAnalysis does not handle nesting of router functions, also the integration test App27 only tests very simple router function definitions.

• The competition library springdoc-open-api has the idea of providing some helper predicates to support router functions better. A first attempt has been NamedHeaderPredicate in this library. Investigate this option closer.

• Improve support for Content-Type and Accept header based content negotiation as we do for WebMVC handler methods

• Improve finding the response code of a router function, see RouterFunctionHandlerMethodMapper.ResponseMapper