added support for dynamic transform endpoints

This allows fcr:transform endpoints to be dynamically created. Instead of using a static definition:

    to("fcrepo:localhost:8080/rest?transform=foo")

or configurable properties:

    to("fcrepo:localhost:8080/rest?transform={{transform}}")

... both of which require restarting the route in order to change, this would allow routes to set headers dynamically, while still permitting a default fallback:

    .setHeader(FCREPO_TRANSFORM)
      .xpath("/rdf:RDF/rdf:Description/indexing:hasIndexingTransform/text()", ns) 
    .to("fcrepo:localhost:8080/rest?transform={{defaultTransform}}")