Add existing Streams as drop-down menu options in "Route to stream" pipeline rule

## What?

Currently when using the "Route to stream" function in the new Pipeline rule editor the user still has to manually copy/paste their existing Stream names/IDs into the respective fields, which means having yet another Graylog tab open in order to accomplish the task at hand. It would be handy if a list of existing Streams appeared here as options instead!

Obviously this is not possible for the source code editor version of the pipeline rule editor, so I'm requesting this specifically for the NEW UI whose intention is to make the user's life easier anyway :)

## Why?
To improve the UX by reducing time spent on a task and number of concurrent Graylog tabs open.

## Your Environment

* Graylog Version: 6.1.6