Make the manager field on a plugin object optional

[Callum027]

At the moment, an implementation of the Manager class must be defined in a plugin class.

buildarr/buildarr/plugins/models.py

Lines 110 to 117 in c8a7e69

	manager: Type[ManagerPlugin]
	"""
	Manager class for the plugin.
	Buildarr instantiates an object of this class without any arguments, and
	runs operations on configurations and secrets metadata through the methods
	defined in this class.
	"""

In practice, however, all of the existing managers within plugins are subclasses of the ManagerPlugin class without overriding any of the existing methods, since ManagerPlugin provides methods that hook into corresponding methods on the ConfigPlugin and SecretsPlugin classes.

Make it so that, similar to the cli field, plugins can simply specify None here (or leave it completely undefined), and Buildarr will use the ManagerPlugin class's base methods unmodified.