Rule Builder: Offer drop-down list to choose stream for route_to_stream function

## What?

When using the `route_to_stream` function in the Rule Builder, Graylog could offer the available streams as a drop-down list, instead of having to type a stream name or id.

On a related note, the current form is confusing because both the stream name and id are shown as optional, but one of them is clearly necessary for the function to work.

## Why?

Save users some typing and potential copy/paste errors.

## Your Environment

* Graylog Version: Graylog 7.1.0-SNAPSHOT+97120f0