Evaluate local paths relative to the config file

[Callum027]

Reimplement configuration parsing so that intermediate configuration files are validated by Pydantic once, before being turned back into dictionaries, merged into one structure, and validated again by Pydantic to get the final configuration object.

This has some implications for how multiple configuration files are handled, and the documentation has been updated to reflect this. Realistically, however, the side effects from this are expected to be minimal.

This allows a new LocalPath attribute type to be added, which is evaluated when these intermediate configuration files are loaded, and parsed to be absolute paths relative to the directory containing the actual configuration file the attribute was loaded from.

If a LocalPath type attribute has a default value and was not defined in the configuration, the directory that contains the originally parsed configuration is used to turn it into an absolute path (instead of the current working directory for the Buildarr process.)

The following attributes have been changed to type LocalPath:

• buildarr.secrets_file_path