Prior to this commit, no `ResourceUrlProvider` was configured
in WebFlux (no bean was contributed by the WebFlux infrastructure).
Also, several `ResourceTransformer` instances that extend the
`ResourceTransformerSupport` base class need a `ResourceUrlProvider`
to resolve absolute URLs when rewriting resource URLs. At this point,
no `ResourceUrlProvider` was configured and they could only resolve
relative URLs.
This commit contributes a new `ResourceUrlProvider` to the WebFlux
configuration; this bean can be reused by the WebFlux infrastructure and
application code.
This also automatically configure this shared `ResourceUrlProvider`
instance on the resource chain where needed.
Issue: SPR-17433
fc957e95bb