Create Prowlarr plugin for Buildarr

[Callum027]

This MR adds the initial implementation of the Prowlarr plugin.

Here are some highlights:

• Compatible with Prowlarr v1.0 onwards
• Prints an easy-to-understand error message that recommends setting the API key in the configuration, if it could not be fetched due to authentication being enabled in the Prowlarr instance
• Uses the semi-automatically generated prowlarr-py library for interfacing with the Prowlarr API
• Update requests for most resources are based on the API schemas so the incoming JSON objects are in the format Prowlarr's web client typically produces
• Generic indexer configuration object for managing an indexer of any type (necessary when there are over 500 unique types available, and even ones using the same implementation have unique fields among them)
• Supports Buildarr instance links with the Sonarr plugin when connecting Sonarr applications to Prowlarr
• At least one resource of every type has been tested to work properly (not all resources have been tested, but I'm much more confident they work properly)
• Some improvements in the implementation of the plugin source that are planned to be done on the Sonarr plugin as well
• Full documentation that will be integrated into the Buildarr website