Reader role can't share Saved Searches to specific users only everyone.

## What?
It would be useful for users who have just the reader role to be able to share their own saved searches to other users. Currently they only get the option in the Add Collaborator of everyone, it doesn't list all the Graylog users, they should be able to share it with.

## Why?
Allow users to share Saved Searches with other users without having to ask a Graylog Administrator to do so, saves time and effort on everyone envolved.

## Your Environment
Only using local Graylog accounts, no LDAP, etc. Using the OSS version of Graylog.

* Graylog Version: 6.1.5+e3ae3ce
* Graylog Data Node Version: 6.1.5+e3ae3ce
* MongoDB Version: 7.0
* Operating System: Debian 12, running official container images
* Browser version: Brave 1.75.181 Chromium: 133.0.6943.141 (Official Build) (64-bit)