Pipeline functions list should have ineligible functions filtered out or marked as "not usable"

## What?

The list of pipeline functions (shown when creating or editing a pipeline rule) should have pipeline rules specific to other products (i.e. other than what's currently active) filtered out or clearly marked as "not usable".

For example: an Enterprise environment should not show or allow use of pipeline functions specific to the Security product.

## Why?

If a feature isn't usable it shouldn't be shown.

I thought I had seen something similar for inputs, but haven't had any luck finding the issue/PR.

Side note: having Enterprise and Security items lumped into the Enterprise changelog, without indication of whether they're Security-specific, has been an annoyance over the years.

## Your Environment

* Graylog Version: `7.1.1` (Enterprise)