Add Sonarr instance URL base support

[Callum027]

https://github.com/buildarr/buildarr-sonarr/issues/43

• Add the url_base host configuration attribute.
• When dumping host configurations, use the provided path as the URL base.
• Read and store the instance version as secrets metadata (instead of fetching it in a separate API request when creating the instance configuration). Necessary for future Sonarr V4 support.