New files engine: Prowlarr (#3118)

## What does this PR do?

Gives the user the possibility to search their own prowlarr instances.

Info: https://wiki.servarr.com/en/prowlarr
Github: https://github.com/Prowlarr/Prowlarr

## Why is this change important?

Prowlarr searchs multiple upstream search providers, thus allows to use that functionality through searx.