rest-dsl - Options requests should not be routed

If using rest-dsl and sending in a HTTP OPTIONS then we should just return back the CORS headers, and not route in Camel.

This is also to support the pre-flight HTTP OPTIONS requests, where rest-dsl returns the CORS headers, then the client can send the actual HTTP request afterwards.

Some good details of CORS here
https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS

We should maybe also consider trying to leverage native cors filters from jetty / netty etc if they provide one and use that. Though these filters like rest-dsl tend to just specify that anything is possible.

We could in rest-dsl look at the rest model and know what each url path has of HTTP methods, so we can return that instead of allowing all HTTP methods.

if there is a simple rest-dsl model with only
GET /users/

POST /users

Then the allowed methods is GET, POST for /users and only GET for /users/123.