Refactor: Use pluggy framework to extract Sync to plugins

[glensc]

Why: The logic in Sync has be come complicated.

Extra: Possible runtime speedup:
Using plugins can skip checks of every media by if plugin for some feature not enabled at all.

NOTE: WatchListPlugin is still too complicated: contains two watchlist sync logics

• https://pluggy.readthedocs.io/en/latest/#a-toy-example

[glensc]

side-effect:

1. Pugin all class methods are resolved: method: object = getattr(plugin, name), this results all @cached_property being resolved at register() time.

bug:

1. plugin methods declared with @hookimpl, using default values makes those values being "invisible": they never get assigned a value, their default value is always used.