Add Prowlarr instance URL base support

[Callum027]

https://github.com/buildarr/buildarr-prowlarr/issues/53

• Add the url_base host configuration attribute.
• When dumping host configurations, use the provided path as the URL base.
• Add support for auto-fetching API keys from newer versions of Prowlarr using the initialize.json endpoint.
• Add support for auto-fetching API keys when dumping instance configurations, by simply pressing Enter without specifying an API key at the prompt.
• Read and store the instance version as secrets metadata (instead of fetching it in a separate API request when creating the instance configuration).
• Use the configured Buildarr request timeout for Prowlarr API requests.