fix(#755): split http-whiteboard out of the camel-cxf feature

[jbonofre]

Summary

camel-cxf unconditionally required http-whiteboard (and therefore a
running pax-web/Jetty instance), even though that's only needed for the OSGi
Servlet Whiteboard-based CXF servlet registration added in #687 (the
"Available SOAP Services" listing page). Producer-only usage and consumer
usage via another transport never needed it - CamelCxfITest already proves
camel-cxf works fine with a manually installed Undertow transport instead,
and camel-cxf-jetty provides its own embedded-Jetty transport, so both were
paying for a Jetty-backed OSGi HTTP Service they don't use.

This follows the same pattern already used for camel-cxf-jetty and
camel-cxf-spring: pull the extra dependency out of the base feature into
its own feature, camel-cxf-whiteboard (camel-cxf + http-whiteboard),
for anyone who wants the whiteboard-registered servlet.

Does this regress a bare-camel-cxf consumer that relied on the default transport?

No. Verified directly while building the test for this PR: publishing a
cxf:// consumer with no explicit transport engine (no Jetty, no Undertow)
fails identically before and after this change, with
HTTPTransportFactory: Cannot find any registered HttpDestinationFactory from the Bus. The whiteboard-registered servlet from HTTPTransportActivator
was never sufficient on its own to publish a CXF endpoint - CXF's default
HTTPTransportFactory still needs a real HttpDestinationFactory (the
JettyDestinationFactory/UndertowDestinationFactory SPI beans that
camel-cxf-jetty/an Undertow transport bundle provide) regardless of
http-whiteboard. So removing http-whiteboard from the base feature
doesn't take away any working capability; it only stops forcing a Jetty
instance that a bare camel-cxf install couldn't actually make use of
either way.

Changes

• Remove http-whiteboard from the base camel-cxf feature.
• Add a new camel-cxf-whiteboard feature (camel-cxf + http-whiteboard).
• Add a Pax Exam integration test (camel-cxf-whiteboard-test) that installs
  the new feature alongside a CXF endpoint served over another transport
  (Undertow, same approach as CamelCxfITest) and verifies there's no
  conflict.

Test plan

• CamelCxfWhiteboardITest (new) passes.
• CamelCxfITest and CamelCxfJettyITest (existing) still pass
  unchanged, confirming the split doesn't break either.

Fixes #755