Add @Symbol to ThrottleJobProperty to expose in declarative pipeline `options`

When using the Declarative Directive Generator for the `options` block, there's a throttle option that corresponds to the throttle step, but nothing seems to be exposed for the throttle job property. Given we prefer to define our config in code whenever possible (leaves a papertrail), it would be nice to be able to define the "Throttle Concurrent Builds" job property in our `Jenkinsfile`s as well.

From what I can tell, the solution here would be very similar to what's described in https://issues.jenkins-ci.org/browse/JENKINS-50561